Your answer is hiding in your comment. You wrote a Manifest v3 extension instead of a v2. If Firefox, an underdog, wants to maintain compatibility, they're going to have to adopt v3. However, they don't want to adopt v3 with the blocking choices Google has made. So they had to do this, unless they wanted Firefox to fade into further irrelevance through extension developers refusing to support it.

Also, technically, Firefox is supporting both 2 and 3, their solution here isn't magical wizardry. It's just maintaining those specific features (blocking Web Requests) on top of adopting 3.