Clean-up system extensions shipped in Firefox 52

In addition to pdfjs there are 6 system add-ons shipped in Firefox 52 looking at the current beta branch. (on release there are 5 right now). We should investigate whether we need all of them and if not make sure that they don't interfere with Tor Browser.

Of particular interest are aushelper and e10srollout I guess.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information