Google has decided to stop releasing the Google toolbar for Firefox. As such, while it may work for some people, there is no support for it, and it is best to start planning for alternatives. https://support.mozilla.org/en-US/kb/google-toolbar-not-compatible-firefox has some of those known alternatives.

But that doesn't explain why it worked and then disappeared.

What version of Firefox were you using before upgrading?

Version 6.

Unfortunately since Google is not maintaining the extension anymore, it is likely they didn't have it set to be compatible with newer versions. Upon upgrading, the Google toolbar would be disabled. You can try installing this add-on https://addons.mozilla.org/en-US/firefox/addon/checkcompatibility/, and see if it brings the toolbar back.

Keep in mind however, that continuing to use the Google Toolbar without support means you are open to both security holes and other bugs (ones that could potentially cause you to lose data) as Google won't fix bugs in the future. https://www.google.com/intl/en/toolbar/ff/index.html

I had the same problem and find on web no solution.

For me worked delete two files in directory GoogleToolbarData: searchhistory.xml and features.properties. Firefox shouldn't be started as you delete these files.

Directory GoogleToolbarData is in your Mozilla/Firefox/Profiles/<your profile> and is OS-depending.

WinXP: C:\Documents and Settings\<user>\Data aplikací\Mozilla\Firefox\Profiles\<profile>.default\GoogleToolbarData

Win7: C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>.default\GoogleToolbarData

I deleted the files, but still don't have the toolbar. Any suggestions?

do you have Google Toolbar in Menu/View/Toolbars ?

No, it has gone away, along with the toolbar.

You have another problem than I had. I had Google Toolbar in Toolbars, but it does nothing if I activated it or deactivated. Google Toolbar wasn't to see.

Sorry, I cannot help you. :-(