You know that I profess another way -- ie. "setting or removing SeaMonkey" instead of "setting or removing Firefox". AFAIK it is kinda "Vivaldi way", when by default you mimic to the well-known browser, and only for some particular sites still use your "real" UA.
So, basically, we should preserve the options implemented in bug #1242294, which allow users to choose the "default" UA way (either FF+SM or FF only). Then the new generic override mechanism should be implemented over it. IOW kinda two-level overriding (first for defaults, second for some known sites).
For example, if you still prefer FF+SM upstream, then you collect "all the banking" sites and provide "FF only" for them. If (like in Fedora) you prefer FF only, then the banking is OK "by default", but you should preserve "SM" for correct work of sites like "firefox.com" (accounts), "thunderbird.net" (addons), "seamonkey-project.org", and maybe also sites that are tolerant of UA strings (duckduck.go etc, you can derive such a list from Vivaldi) for statistical needs (if any).
Depending on what default UA is used, distribution can use either upstream's or its own json file for the overrides.
It might be a good idea to make these overrides possible for mail domains as well (it will not be surprising if some large mail site forgets about SM existence completely and starts marking mails as a spam due to UA header string).