Convert `FirefoxCom` to a class, with `static` methods
Convert `FirefoxCom` to a class, with `static` methods

*Please note:* It's highly recommended to ignore whitespace-only changes when looking at this patch.

Besides modernizing this code, by converting it to a standard class, the existing JSDoc comments are updated to actually agree better with the way that this functionality is used now. (The next patch will reduce usage of `FirefoxCom.request` significantly, hence the JSDocs for the optional `callback` is removed to not unnecessarily advertise that functionality.)

Finally, the unnecessary/unused `return` statement at the end of `FirefoxCom.request` is also removed.

Snuffleupagus/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,caremerge/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,timvandermeij/pdf.js,xavier114fch/pdf.js,macroplant/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,nawawi/pdf.js,caremerge/pdf.js,macroplant/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,Snuffleupagus/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js