Add /firefox to the list of pages we deliver enhanced to legacy browsers

[hoosteeno]

Description

In #7389 (comment) we agreed to remove IE9/10 from the list of browsers getting enhanced support. We also agreed to add /firefox to the list of pages where we try to provide enhanced support for browsers going all the way back to IE6.

This bug is about adding /firefox to that list.

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💛 Success Criteria 💛

• /firefox continues to work well in IE9/10 after we move them to the list of browsers that get a more basic experience

❗ Risks ❗

• This is an example of a risk. Use a checklist!
• This is an example of a risk.

Tasks

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[alexgibson]

Going to close this as a dupe of #7559 (adding /firefox to the excluded pages is a sub-task there, and makes sense to do at the same time).