Second, both projects have been in active development for years so I'd imagine their userbases are substantial. Pentadactyl has about 20k downloads for release 1.0 alone, probably closer to 100k for all versions. Certainly not the majority of FF users but hardly ignorable either.
Australis was in the pipeline for a long time, and all work was done in the open. It's one of the reasons there was such a huge fuss made about it.
Addon/extension authors had plenty of warnings and opportunities to make sure that their code didn't break.