Safe mode is mainly for troubleshooting. In safe mode, you cannot use any addons and I don't think it is good for practical usage of Firefox. I don't have any problem with 3.5 but, if you have problems in your environment, it would be appropriate for you to wait till the developers solve the problems.
Personally, I use portable version of Firefox, which allows me to test a new version without affecting on my old setup. Some other people probably use different profiles for the same purpose. In any case, those people have their ways to avoid risks. This is why I twice or thrice recommended FF users to use portable version or at least a profile under a different name while testing a new version. Reading quite many posts complaining on the new version of Firefox, I think that Mozilla have their share of responsibility in promoting Firefox to the lowest common denominator. Recommending a new version is one thing and letting them know what they may lose with the new version is another.