I’m not sure about these stable packages, but for the nightly packages they introduced recently they explicitly mention on [1] that you can keep browsing:
> Following community discussions, we have updated the post to highlight that Firefox can continue browsing after an APT upgrade, allowing people to restart at their convenience.
Oh, that's great! Between that very clear statement and the fact that in this post they call out that "you will still need to restart Firefox for the latest version", it sounds like they've finally fixed it!
> Following community discussions, we have updated the post to highlight that Firefox can continue browsing after an APT upgrade, allowing people to restart at their convenience.
[1]: https://blog.nightly.mozilla.org/2023/10/30/introducing-mozi...