Mozilla Firefox development is coming along nicely. Nightlies are now showing a version bump in anticipation of the 0.10 release.
About once a week I install a new nightly, and must say that using the bleeding edge builds hasn’t caused any major problems.
Word of caution though if you want to try a nightly but you currently use a build labeled as .098 or lower. Not all extensions are compatible with the new version. Be sure to check your extensions before you install unless you can live without certain extensions.
The two extensions I use are now both compatible.