After upgrading firefox to 3.5.2 version I noticed that there is no tooltips. Tooltips are missing and instead of missing tooltip there was small white rectangle cursor box. I found solution for this problem with a help of google.
It is no tooltip bug (or blank tooltip bug) in Firefox 3.5. Solution is to uninstall and reinstall the latest version of Google toolbar for Firefox. To be sure that problem is in Google Toolbar try to start Firefox in safe mode. If Firefox in safe mode properly display tootips then source of problem is in one of the add-ons (probably Google Toolbar for Firefox)
To uninstall Google toolbar for Firefox click Tools-->Add-ons, choose on Google toolbar and click Uninstall
This article refer to no tooltip bug or blank tooltip bug in Firefox 3.5.