Are you sure it's IE 7? If you do have IE 7, is there some reason why? Latest is IE 8, maybe updating IE to the current version would possibly help.
Another thing, you could uninstall, and reinstall firefox. And check your firefox add ons, if you have any. One or more may be causing a conflict. Disabling them one by one, re create the situation where the problem occurs, and see if it happens or not with them disabled.
Also, can you post a list of what Firefox add ons you are using?
And BTW, Firefox 3.6 final is supposed to be released tomorrow, the 21st.
So possibly installing 3.6 and if your IE is IE 7, then updating that to IE 8 as well.