I've used both Firefox and Thunderbird, but ended up 'moving back' to Seamonkey. My reasons for this move were the rather heavy load the combination of FF and TB puts on my - older - hardware as well as the fact that SM is more or less up to par with FF and TB while offering some 'convenience' functionality (eg. CTRL-2 opens mail from within browser, CTRL-1 opens browser from within mail). Another advantage SM has is that it supports the 'original' FF sync (which I use in combination with the Owncloud 'Mozilla Sync' app) without the need to jump through hoops. It also has a 'normal' preferences dialog, normal tabs, normal... everything. In other words, it did not succumb to the 'Chrome copy' trend which FF embarked upon.

Seamonkey happily provides me access to my mail (several accounts), dozens of open tabs in several browser windows and more in about 500MB of RSS. Firefox and Thunderbird together would use almost double that. While this might not matter on a recent system with 8+GB of memory, on this Thinkpad T42p with a maximum of 2GB it is a significant difference.