IETab is a nifty extension that allows you to view pages in Firefox, while rendering the pages using the Internet Explorer rendering engine.
Unfortunately, IETab, since version 1.0.7, does not play well with Back IS Close. If IETab is installed prior to Back IS Close, the Back button will completely disappear.
In order to use both Back IS Close and IETab, you must install Back IS Close BEFORE installing IETab
If you already have Back IS Close installed and you want to install IETab, just go ahead and install IETab.
If you already have IETab installed and you want to install Back IS Close, first remove IETab and re-start Firefox. Then install Back IS Close and restart again. Then, re-install IETab and restart again..
You do not need to uninstall IETab if you're only updating Back IS Close.
To check if you've installed both extensions in the right order, go to Tool | Extensions in Firefox to display installed extensions. In that list, Back IS Close should be listed first (above) IETab.