Firefox Devtools MCP vs Feedthrough

connection_status

Check whether a browser with the Feedthrough bridge is currently connected. Returns connected flag and a list of open tabs (id, url, which is active). Call this first — every tool except get_instructions requires a connected browser.

get_console_logs

Return console output captured since the bridge connected. Covers every console method — log/warn/error/info/debug plus dir, table, assert, trace, count, countReset, time/timeEnd/timeLog, group/groupCollapsed/groupEnd, and clear. Each entry has a 'level' (the closest of the five standard levels); rich methods also carry a 'method' field, and console.trace() plus failing console.assert() entries include a 'stack'. Uncaught exceptions and unhandled promise rejections are also captured (level 'error', method 'uncaught' / 'unhandledrejection') even though the app never logged them. When the app is noisy with framework or deprecation warnings, pass levels: ['error'] (or ['error', 'warn']) so the real errors aren't buried, and use 'match' to narrow by content. Pass 'since' (a ms timestamp from an earlier entry's 'ts', or Date.now() before an action) to see only what happened after that point. Read-only: it returns a passively captured buffer and neither clears the console nor changes the page. Always check this early — app errors and debug output often identify the root cause immediately.

get_network_requests

Return all fetch and XHR requests captured since the bridge connected, including URL, method, HTTP status, duration, request and response headers, and request and response bodies (bodies capped at 10 KB each — anything longer is truncated with a marker; binary responses are summarised). Use this to find failed requests (4xx/5xx), wrong URLs, slow calls, or to inspect what the app actually sent or received. Use 'filter' to narrow by URL/method and 'since' (a ms timestamp) to see only requests that fired after an action. Read-only: it returns a passively captured log and does not issue or modify any requests.

query_dom

Query the page with a CSS selector and return a summary of every matching element (tag, id, classes, text content). Good for counting list items, checking what's rendered, or finding the right selector before calling inspect_element or click. Read-only: it only reads the DOM and never changes the page. Returns an empty list (not an error) when nothing matches, so it is also a safe existence check.

inspect_element

Return full details about a single element: tag, id, classes, all attributes, text content, bounding rect (top/right/bottom/left/width/height + page scroll and an inViewport flag), a compact ancestor 'path' (e.g. 'body > main > div#app > button.cta'), a curated set of computed styles (layout, box model, typography, positioning, flex/grid), an 'overflow' block when content is clipped/overflowing (scroll vs client size + per-axis x/y flags), a 'clipped' block when an ancestor's overflow cuts the element off (the clipping ancestor + which edges), an effective-visibility check ('visible' boolean, with a 'hiddenReason' such as 'ancestor div#modal display:none' or 'opacity:0' when not visible, accounting for ancestors), an occlusion check ('hittable' boolean from a center-point hit-test, with 'occludedBy' naming the element actually on top when something covers it), an 'a11y' block (resolved role, best-effort accessible name, and key states like expanded/checked/selected/disabled/hidden/tabindex), a 'pseudo' block with ::before/::after content when set (icon fonts, generated text), and live form state where applicable (an input's current value, checked, disabled, etc.). Pass 'properties' to additionally read any specific computed CSS properties by name — they come back under 'requested'. Use this to understand why an element looks wrong or isn't behaving as expected. Read-only: it only reads element state and never changes the page, and it returns an error if the selector matches nothing. Note: addEventListener-registered event handlers cannot be read from the page; only inline on* handler attributes appear (in 'attributes').

click

Click an element by calling its native click(), which fires a click event and runs the default activation: following a link, toggling a checkbox or radio, submitting a form. Prefer an id selector (#submit-btn) for reliable targeting. Note it does NOT synthesize the preceding pointer/mouse sequence (pointerdown / mousedown / mouseup) or move focus, so a handler wired specifically to those events rather than to click won't fire; for keyboard-driven activation use press_key instead. Behavior: if the selector matches nothing the call returns an error; it does not scroll the element into view, and it does not wait for any resulting navigation, network, or re-render to settle, returning as soon as the click is dispatched. Observe the effect with a follow-up get_console_logs / get_network_requests / query_dom. Returns the tag and id of the clicked element.

fill

Set the value of an input, textarea, or select element. Focuses the element, assigns the value through the element's native value setter (so React/Vue controlled inputs register the change), then fires bubbling input and change events. The value is set in one shot, not typed character by character, so per-keystroke handlers (keydown / keypress / keyup / beforeinput) do NOT fire; to send Enter to submit or trigger a key shortcut, follow with press_key. Prefer an id selector (#search-input). If the selector matches nothing the call returns an error; it returns as soon as the events are dispatched and does not wait for downstream validation or re-renders. Returns the tag and the value that was set.

hover

Hover over an element by dispatching synthetic, bubbling mouseover and mouseenter events from inside the page. This triggers JavaScript hover handlers (onMouseEnter / onMouseOver), so hover-only UI that mounts on hover (tooltips, popovers, dropdown and submenus) appears in the DOM; follow up with query_dom, get_html, or inspect_element to read what was revealed. Three limits to know: it does NOT activate the CSS :hover pseudo-class (that is driven by the real cursor, not synthetic events), so styles or content shown purely via :hover in CSS will not change; no mouseout / mouseleave is sent, so the hovered state stays until the app tears it down or you interact elsewhere; and the events are dispatched whether or not the element is visible or in the viewport (it is not scrolled into view), so a successful call does not by itself confirm anything rendered. If the selector matches nothing the call returns an error. Returns the tag of the hovered element.

press_key

Dispatch a key press (keydown/keypress/keyup) on an element — e.g. Enter to submit a search, Escape to close a modal, Tab to move focus, or ArrowUp/ArrowDown in a list. Use named keys (Enter, Escape, Tab, Backspace, Delete, ArrowUp/Down/Left/Right) or a single character. Note: this fires key handlers but does NOT insert text into inputs — use 'fill' to set an input's value, then press_key for the submit/shortcut. If the selector matches nothing the call returns an error; it dispatches the key events and returns without waiting for any resulting navigation or re-render.

get_html

Return the outerHTML of an element (capped at 50 KB). Use this when the summarised query_dom output isn't enough and you need to see the actual markup/structure of a region. Read-only: it only reads the DOM and makes no changes, and it returns an error if the selector matches nothing.

get_page_info

Return basic page context: current URL, document title, readyState, viewport size, scroll position, and user agent. Read-only and non-destructive: it only reads page state and makes no changes. Useful to orient at the start of a session or confirm a navigation happened.