That it is.. though I do not use the release builds as they are to old for me as i primarily use the Branch nightlies as I need stability over potential crashing etc in the unstable Trunk.
Firefox 1.5.0.1 UA is for example Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.0.1) Gecko/20060111 Firefox/1.5.0.1 so in a ways it has been out for a while.
Nightly/beta builds are pretty safe if you are careful not to use your precious Release build profile with a nightly build so as to not corrupt the profile. Just install in a separate place with a separate profile made for that build with the profilemanager. The new updator nowdays makes this easier/quicker/lazier though