Mozilla has an about
:config page where you can alter many advanced or hidden settings. Most of them are not interesting, and those that are can generally be configured through the normal settings panel. Just browse to "about
:config" (I didn't type that space).
browser.chrome.toolbar.tips has true as its default, so somehow it got altered at digiots setup. I didn't know about the setting, but I'll keep it in mind if the pop-ups stop working. :-)
About browser.display.force_inline_alttext, it's not for making the ALT attribute a pop-up. Here's an quote from Hidden Mozilla Prefs
// This pref displays the alt-text instead of the "image-not-found" image
// true = force ALT text for missing images to be layed out inline
I have found the plugin that can be used to make the ALT attribute pop-up, it's over here for both Mozilla and Firebird: