docs: replace outdated IRC link with Mozilla Matrix channel

[tenderdeve]

Description

The debugging tip in docs/add-to-firefox.md referenced the #webextensions IRC channel on irc.mozilla.org. Mozilla shut that server down in 2020 and migrated to Matrix, so the link was a dead end for anyone following it.

Replaced with the current #webextensions room on the Mozilla Matrix server.

Changelog

CHANGELOG entry: null

Related issues

Fixes: #42178

Manual testing steps

1. Open docs/add-to-firefox.md on the PR branch.
2. Confirm the IRC reference is gone and the Mozilla Matrix link https://chat.mozilla.org/#/room/#webextensions:mozilla.org renders correctly.
3. Click the link and confirm it opens the #webextensions Matrix room.

Screenshots/Recordings

N/A — documentation-only link change.

Before

#webextensions channel on irc.mozilla.org (dead server since 2020).

After

#webextensions room on https://chat.mozilla.org/#/room/#webextensions:mozilla.org.

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable (N/A — docs only)
• I've documented my code using JSDoc format if applicable (N/A — docs only)
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[github-actions]

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