Why does Firefox suck all of a sudden?

After years of dealing with the crappy Explorer browser, it was a relief to switch to Firefox several years ago.

Now Firefox is starting to suck as well. I routinely get a notice that says “The URL is not valid and cannot be loaded” when I hit a link ot try to go to a site, even though everything is fine. Close the box and try again (sometimes several times) and the link works fine.

Can somebody please fix this? It’s annoying as hell . . .