Firefox doesn't delete incompatible extensions, it disables them. Right-click that extension (in the Addons > Extensions tab) and use Find Updates to see if Yahoo made use of that feature in the extension manager. If not, you'll need to locate a Firefox 4.0.x compatible version and install it.

I went the Tools Menu>Add Ons>Extensions>Scrolled down to the Yahoo Tool Bar>Double clicked on it, made sure that the option button 'ON' was marked, (mine was on default) Clicked on the 'toolbar.yahoo.com' link and downloaded the the updated version of the yahoo tool bar...Viola! My 'took forever to set up' yahoo toolbar appeared...  ;-D