Issues with tooltips not working properly have been reported to be caused by the Google Toolbar (Tools > Add-ons > Extensions)
You can check if there is an update available that fixes that issue.

You can try to uninstall and reinstall the latest GT, that has been reported to work.

• http://www.google.com/tools/firefox/toolbar/FT5/intl/en/
• http://tools.google.com/tools/firefox/toolbar/FT5/intl/en/install.html

See also Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

On mouse over I see a little image instead of a label. This occurs on Firefox, IE but not on Safari.

I had the same problem, but noticed that it occurred only on my secondary monitor. Correcting the problem with the drop-down menus corrected this one as well.

Tools > Options > Advanced >Clear the "Use hardware acceleration when available" option.