At enterprises its not uncommon that even dot releases break some apps.
In my experience (nearly 15 years in enterprise development) this isn't the case at all. It used to be the case, especially with apps that used plugins or Java (both of which were common in the early 2000s), but for the most part these apps have gone now.
I think it is very, very unlikely that any non-client-Java webapp that runs in Firefox 3.x will break in any significant way in any future release.
In my experience (nearly 15 years in enterprise development) this isn't the case at all. It used to be the case, especially with apps that used plugins or Java (both of which were common in the early 2000s), but for the most part these apps have gone now.
I think it is very, very unlikely that any non-client-Java webapp that runs in Firefox 3.x will break in any significant way in any future release.