i use ff as well as ie. need to test css in both
firefox: +++
tabbed browsing
extensions
full css2/3 support.
etc
ie: +++
fast
integrated in windows
firefox ---
slow - takes ages to load and slows down with a few tabs
big memory hog
ie --
no tabs
as for ie7, i stay away from beta products. comp tends to get slow cos of constant installation/uninstallation.
i keep ghost copies of my basic installation but even then recreating the whole system as i like it can be borring over and over every 3-4 months or so.