When you say "reopen", I assume you mean completely close all open instances, wait, then relaunch. If so, it could be related to Prefetcher, the predecessor to Vista/7's Superfetch, which loads parts of commonly-used programs into RAM to reduce load times. In this case, it's something that may be sorted out naturally as Windows analyzes your usage habits.

Also, both Firefox and Thunderbird use extensions, some of which may be shared, that can slow load times considerably. After Firefox/Thunderbird is launched and closed, those elements may remain in memory for a short period, reducing the load time if you relaunch the programs soon after. To test the hypothesis, try running both applications in their respective safe modes, which disables extensions, setting modifications, etc. (Ex: Start->Run->firefox.exe -safe-mode)

John