Toggling it from the menu brings back the bookmarks for me as well, but this isn't a solution.

I SOLVED THIS PROBLEM and many others listed below. WHAT FINALLY WORKED: Windows. I went to Control Panel>Add/Remove Programs, right-clicked Mozilla Firefox, and clicked REPAIR. Windows repaired the installation, and now everything is back to normal.

I had this problem, and a smattering of others that were equally annoying, all at once. First, the bookmarks toolbar would be visible (the toolbar itself, not any bookmark icons), and it was checked under View>Toolbars. Unchecking that and then rechecking it, or more simply pressing Ctrl+/ to pop-up the Add-Ons toolbar and then remove it both restored the icons, but that wasn't fun.

Secondly, I couldn't open any new tabs, either with the New Tab (+) button, with Ctrl+T, or by right-clicking a link and clicking Open in New Tab. Open in New Window worked fine, and all of my settings for tabs had not changed at all, but changing and restoring them did nothing anyway. The only circumstance under which I could get multiple tabs to open would be with a bookmark folder on the toolbar, clicking Open All in Tabs, which did the trick.

However, that had the nasty side effect of graying out my Menu bar so I couldn't select any of the menus, and also making it impossible to close any of the tabs. The same thing happened when I right-clicked a toolbar and went to Customize.

Many solutions on these pages did not help, including anything related to the Ask Toolbar, restoring toolbars to default settings, disabling Add-ons/Extensions, Safe Mode, deleting and restoring the localstore.rdf and places.sqlist files, anything in about:config, etc. I didn't have the option of uninstalling and reinstalling either, as I am on a managed machine.

I assume it was related to a combination of updating to Firefox 4 and some extension or add-on, but that worked for me without messing with my bookmarks or settings in any significant way. GOOD LUCK!