Outdated IRC debugging link in add-to-firefox.md — Mozilla moved to Matrix

Nobody has claimed this yet.

### What is this about?

## Description
The file `docs/add-to-firefox.md` recommends connecting to the IRC channel
`#webextensions` on `irc.mozilla.org` for debugging help.

Mozilla shut down its IRC server in 2020 and migrated to Matrix.
This link leads nowhere and will confuse developers seeking help.

## File
https://github.com/MetaMask/metamask-extension/blob/develop/docs/add-to-firefox.md

## Suggested fix
Replace the IRC reference with the current Mozilla support channel.
The correct resource is now the Mozilla Matrix server:
https://chat.mozilla.org/#/room/#webextensions:mozilla.org

Or simply link to the Firefox Extension Workshop support page:
https://extensionworkshop.com/

### Scenario

_No response_

### Design

_No response_

### Technical Details

_No response_

### Threat Modeling Framework

_No response_

### Acceptance Criteria

_No response_

### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

_No response_

Open docs/add-to-firefox.md and find the outdated #webextensions IRC reference. Replace it with the current Mozilla Matrix room or the Firefox Extension Workshop support page, then check the rendered Markdown and verify the selected link works. Done means the document no longer directs developers to the retired IRC server.

Written by the indexing model from the issue text.