If Firefox freezes up for a few seconds every time you try to download a file, it's probably caused by an excess of old completed downloads left in the downloads manager. There is a short and simple solution to this problem…
You can usually fix this problem by opening the Downloads window(Tools > Downloads -or- hold Ctrl and press J), and clicking the Clean Up button. This will clear that long list of old downloads.
Once you've done this you shouldn't have that pesky freezing problem anymore.
While I'm talking about the download manager, I should recommend the Download Manager Tweak extension, which puts a cleaner look on the downloads window, and adds a few handy buttons, like Open Folder, to open the containing folder for a download.
It’s gettin’ there…
Shantanu Oak