EDIT: Apologies. Updated with a link to what gorhill REALLY said:
Manifest v2 uBO will not be automatically replaced by Manifest v3 uBOL[ight]. uBOL is too different from uBO for it to silently replace uBO – you will have to explicitly make a choice as to which extension should replace uBO according to your own prerogatives.
Ultimately whether uBOL is an acceptable alternative to uBO is up to you, it’s not a choice that will be made for you.
Will development of uBO continue? Yes, there are other browsers which are not deprecating Manifest v2, e.g. Firefox.
Are you sure that it was Firefox itself? I find the few times something like that has come up, it was because of extensions (like adblocl, actually).
Delta’s website started blocking me due to using Dark Reader, apparently something about detecting that the contents of the page were being altered. And another site worked fine when I disabled unlock; I assume because it was blocking loading some .js that was actually being used for something other than just ads.