For a few days now, when using Firefox a pop-up appears which is extremely irritating; it says, "Would youlike to share your location with this site?" s0.2mdn.net
It pops up most persistently, giving me oly a few seconds respite when I tell it to go away. Help!
I don't personally like Firefox and Safari works very well (IMHO). Given that it's sitting there on your computer, why not see if it has the same problem?
2mdn is apparently an advertising site that installs an extension. If you don't want to part with FF, you could try the advice here (that amounts to looking in FF's extensions for the culprit - I expect all the scary reboot stuff is only necessary for Windows). Hopefully on the Mac version of FF it has not succeeded in getting beyond being a simple FF extension.
As a wild guess, have you recently updated Java? That now seems to offer advertising extras (from Yahoo) which is rather irritating.