Thanks for the replies, everyone. I did already try changing settings of all kinds, however. This is something that just happens out of nowhere, and isn't connected to settings in the control panel or in Firefox "View" or resolution or text size or personal settings or anything else. Suddenly everything on the screen starts either shrinking into oblivion or enlarging to utterly enormous.
I play Spider Solitaire and that's about it for games.
I'm starting to think it's my wireless mouse which is acting goofy...or maybe it's triggered by using Macromedia Dreamweaver or Fireworks. Whenever I go into Fireworks, I get a message that the system is going to alter the color settings because the software is a generation or two old. I haven't correlated the funny magnification-spaz problem to using Fireworks, though. Will have to try it, I guess.