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.
What are the main differences, for someone considering going from Firefox to Librewolf?
Librewolf is to Firefox what Chromium is to Chrome, essentially. Removed many bloated Mozilla anti-features, has sensible (but not paranoid) privacy and security defaults and ships with uBlock origin pre-installed. You can archive all of that with Firefox, but Librewolf makes things easy for you.
the first comparison is not technically correct, in the sense:
LibreWolf implements additional privacy features and settings on top of Firefox. Chromium is the base browser that everyone else built on top of. It does not implement additional privacy features.
perhaps a better comparison would be: LibreWolf is to Firefox what Ungoogled Chromium is to Chromium
Librewolf is hardened out of the box whereas with Firefox, you have to harden it yourslf
You should also consider arkenfox user.js