If I upgrade to 6.0, OK to continue to run 5.0 w/Librarian?

As far as I know, there is no problem to use FF 6.0 and keep using FF 5.0 because the upgrade is really not an upgrade, it is really a totally new software that is independent from any other version

The licenses are each one separate for each version, and the librarian can be only used with the version you have, so you can not use librarian 5 with 6

I have FF 4.0, FF 5.0 and FF 6.0 in the same computer and same hard drive without problem, they do not share the filters or any other files

I do not know about Librarian, but I think you will keep using it with FF 5.0