I have been a loyal mozilla user since about Milestone16, but have been using Firefox almost exclusively since it was first renamed to Firebird from Phoenix. I find Firefox loads much faster, has a lot more extensions and themes available, and is generally much nicer to use than Mozilla. The same is true for Thunderbird IMHO.... the only thing I find lacking is the integration between the two and the lack of standalone "Composer" application. I definitely vote for Firefox/Thunderbird!