You seem to have installed an extension that requires the Firebug extension to be installed.

• http://getfirebug.com/wiki/index.php/Firebug_Extensions#Page_Speed

If you do not use either of those then uninstall (remove) the not working Page Speed extension.

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

I have this exact same problem, the only difference is that the message appears off the screen and won't disappear when you click it.

Also when I go to check my extensions to sort page speed, firefox tells me I don't have any extensions of this type.

If I don't click OK on the message, I cannot open tabs. However I cannot disable the extension to remove the message in the first place.

I have firebug and page speed installed. The Firebug and Page Speed apps work just fine but Firefox tells me I have neither of them installed.

This can be a problem with the file(s) that store the extensions registry.

Delete the files extensions.* (extensions.sqlite, extensions.rdf, extensions.cache, extensions.ini) and compatibility.ini in the Firefox profile folder to reset the extensions registry.
New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is an update.