docs: update firefoxPreferences.js reference

[emmanuel-ferdman]

Your checklist for a pull request to sitespeed.io

Please review the guidelines for contributing to this repository.

• I'm making a big change or adding functionality so I've already opened an issue proposing the change to other contributors, so I got feedback on the idea before took the time to write precious code
• Check that your change/fix has corresponding unit tests (if applicable)
• Squash commits so it looks sane
• Update the documentation https://github.com/sitespeedio/sitespeed.io/tree/main/docs in another PR
• Verify that the test works by running npm test and test linting by npm run lint

Description

Small PR - adjusts the firefoxPreferences.js reference in docs/documentation/sitespeed.io/browsers/index.md.

[soulgalore]

Thank you @emmanuel-ferdman !