Text displaying selectively, and programs not letting you input text

Question

I posted here once before under a different name when my internet wasn't working and got a prompt, exceedingly detailed and very useful response. Hopefully another brilliant Mac user can help me this time!
As you can see I have an intel-core solo Mac mini. I love it very much, but recently text has been display in an unusual way. Sometimes, in some programs, certain letters will not display. http://tinyurl.com/ygmhc4 is a collage thing I just made to show what I mean more clearly. This happens in MSN, in system. . .programs, I guess, in Firefox, in Growl notification, in Google Notifier, all over the place! I haven't noticed it in MS Word yet, though, but I don't use it often. It's really quite annoying. Any thoughts? It tends to be different letters, but for example all lower-case Us will disappear. But next time, it will be all capital Hs and lowercase Fs. And if you close the program and reopen, it sometimes displays correctly! In Firefox, sometimes entire passages of text will not appear, unless I change the text size to very small sizes. How perplexing!
And another issue I have had: Sometimes a program like Firefox will not recognize ANY keyboard input unless I switch to another application, and then back again. It's a minor annoyance, but it happens pretty regularly.
Help with either issue would be GREATLY appreciated as while both are not destroying my machine, they are harming my productivity. Thanks SO MUCH!

Answer 1

Oh and another display issue, although this MAY be a Firefox problem as it only happens in Firefox. Then again, it seems my computer is really having trouble with text.
Here it is: when I am entering text in to a textbox, the text-entry cursor is slower than the text I am entering. For example, see http://tinyurl.com/yakrlp
In that box, I typed "Unfortunately, this is not the end of my crazy text-display issues! I can't see the last couple of letters I've typed until I type severl mare after them causing ridiculous typos like those because It'it's too late! And I dont't know how many backspaces to do because the cursor is in the wrong place. Annoying, huh? Let me tell you it is. Perhaps this is a Firefox issue, but it seems like it isn't."
As you can see, this is causing a lot of problems. It only appears on SOME websites. For example, on this site, entering this text, it's fine. On some forums, like the MozillaZine support forum, and also in Gmail, this problem appears. Please help! Or is this a Firefox issue?
It seems I have a lot of problems!