[TomGuy]

I saw a previous reference to FF ESR. Does it accept add-ons, etc.?

I used to have an add-on that checked versions for other add-ons, but the developer quit updating it around FF version 16.

I don’t want to try another FF and have it mess up my current version+add-ons.

[ShadowAce]

I saw a previous reference to FF ESR. Does it accept add-ons, etc.?

Yes, it does. It is merely a stabilized version that doesn't update so frequently. I believe it is currently at 17. It does backport the security fixes Mozilla releases.