See what happens in the Firefox SafeMode.

Help > Re-start with Addons disabled or hold down your Shift key as you click the Firefox short cut.. Do not check any boxes. This is a fault finding mode.

When in Safe Mode...

• The status of plug-ins is not affected.
• Custom preferences are not affected.
• All extensions are disabled.
• The default theme is used, without a persona.
• userChrome.css and userContent.css are ignored.
• The JIT/Javascript compiler is disabled.
• The default toolbar layout is used.
• Hardware acceleration is disabled.

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required) when you see this issue?

You can check for problems with preferences.

Delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

For example: https://twitter.com/firefox

I have attached an illustration of the expected mouseover tooltips from Chrome. These are generated by a script, so presumably the script does not run the same in Firefox for some reason.

Thanks to jscher2000 for reproducing and displaying the issue. I have tried running in safe mode and have also performed a complete refresh which would have reset any preferences, but the problem persists.

I have the same problem in Firefox 42. Fails in safe mode in a default profile. I have spent hours searching for info on this problem but no luck.

Seems to still be the same in Firefox 44 on Windows. To be honest, I so seldom use Twitter that I'm not highly motivated to try to solve this one. Sorry.

Well I cited Twitter as an example as its very reproducable for anyone looking into this, but tooltips are so ubiquitous it could be a symptom of a potentially wider problem than just Twitter.