Support handbook: remove Firefox extension references

Lenny Vakninerequested to merge
corpsec-remove-firefox-extension-publishing into main

What does this MR do?

Removes Firefox-related extension content from the Support handbook:

  • Drops Firefox rows from the "Current Extensions" table on the Browser Extensions page
  • Removes the bullet directing developers to publish Firefox extensions via personal Mozilla accounts
  • Removes Firefox links from the "Useful Browser Extensions" list on the Support Engineering page

Why

GitLab's managed-fleet browser tooling and extension allowlisting is centered on Chrome. The Support handbook should not direct team members to publish or install extensions on browsers that are outside the managed-extension scope.

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