In your question, you indicated that you are running Firefox 8. Is that correct? It might be difficult to diagnose issues with that version because most people have moved on to Firefox 12 (plus or minus 1 version). Can you upgrade?

It's hard to think of a reason that ordinary text or links in ordinary text would not display. For embedded images or videos, one possibility is a difference in the protocol, i.e., HTTP (not secure) versus HTTPS (secure).

Hopefully someone else will have a better guess (or actual knowledge!).

Regarding the blue lines in a message, that usually indicates the earlier message was forwarded a few times. I don't know whether Gmail will let you reformat that area or whether you have to clean it in another application (e.g., for plain text, in Notepad) and paste it back in.

Thank you jscher2000 for the quick answer. I upgraded to version 12, I don´t know why Firefox didn't prompt me to do it when it was available, normally it upgraded itself automatically, that's when I lost my useful Google toolbar. But after upgrading, I pulled out from the waste basket the mail that triggered this question, and still do not show the http address (is from You tube) but replying or re sending, shows it. Probably because I received it through the older version of Firefox. I'll wait and see. Thank you again for your answers (in both questions)