I just updated FireFox to ver 57.0 for 64-bit and on restarting found that Avast antivirus had been installed. Not sure whether it was part of the new version, but uninstalled it as I have adequate AV protection, i.e. SuperAntiSpyware, Defender, Spyware Blaster, and MalwareBytes.
Edit: In retrospect, I think it was a pop-up for installing Avast.
Sorry, don't understand this. A tab is just a web page. To "find your old tabs", surely all you need to do is open a fresh tab and navigate to the web page you want.
Still missing an extension you depend on? This Google document is tracking several popular extensions, and provides alternatives to several common ones. This list is also useful.