The Firefox Release Candidate (RC) is a refined build. The nightlies continue to be minefields. I would disable the nightly ppa. Then, download the RC to /opt and symlink
/usr/local/bin/firefox. Once there is an official ppa for the RC releases maybe install that, but the RC from Mozilla should auto-update.
RC vs Nightlies/Dailies:
I deduce, since their launchpad has nothing to say for itself, that the Ubuntu Mozilla Daily team pulls source from Firefox and builds it. That source code is the same foundation for Mozilla.org's Nightly Builds, which you can see is currently labeled by Mozilla.org:
firefox-4.0b13pre. (Possibly I'm wrong, and the Ubuntu Daily Firefox team actually pulls source from mercurial that has passed basic tests. Regardless, it's in the same category as Mozilla.org's nightlies).
The Mozilla.org Firefox Release Candidate (RC) 1 is a nightly build tested, spit-polished, and renamed. It is not an official 4.0 release, which is why you don't see the nightly builds move from 4.0b13 to 4.01 or whathaveyou. The RC has its own source branch (which I'm sure you can get using mercurial), but really won't be hacked on. You can also find the static source for RC1 at the public ftp site.
The next RC will not be built off of the RC1 code branch. It will be another nightly build tested, spit-polished, and renamed Mozilla Firefox RC2.
More about nightly builds:
Mozilla.org has this to say about its nightly builds (thus also the launchpad-mozilla-daily's daily firefox builds):
We make nightly builds for testing
only. We write code and post the
results right away so people like you
can join our testing process and
report bugs. You will find bugs, and
lots of them. Mozilla might crash on
startup. It might delete all your
files and cause your computer to burst
into flames. Don't bother downloading
nightly builds if you're unwilling to
put up with problems.
You'll note that Ubuntu-Mozilla-Daily's last attempt to build Firefox failed.
That said, the nightly builds should be fairly close to the release candidates, for reasons that should now be obvious.