docs note mozilla firefox works for u2f

What does this MR do?

List Mozilla Firefox as compatible, since current releases support U2F (it was added back in Firefox 60)

Related issues

Author's checklist

• [ ??? ] Apply the correct labels and milestone
• [ N/A ] Crosslink the document from the higher-level index
• [ N/A ] Crosslink the document from other subject-related docs
• [ N/A ] Feature moving tiers? Make sure the change is also reflected in features.yml
• [ ??? ] Correctly apply the product badges and tiers
• [ ??? ] Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards