300 -400MB isn't enormous.

How much RAM do you have?
How much is AVG reporting that Firefox is using?

I just got another high-memory usage alert. It was 301MB. So if that's not too much why do I keep getting this message? I don't even have enough disk space to do my automatic back-up.

Disk space has nothing to do with memory usage, it is related to a full hard drive. Between those 2 problems it sounds like you need a new PC, or more RAM and a larger hard drive.

The pop-up Firefox (AVG) warning says that I am using an unusually high amount of memory (235 to 285) and that I should restart FF Notebook RAM is 3.00 GB - CPU speed 1.73 GHz (Intel Pentium Dual) Need help - how to I get to "solved"????

235 to 285 MB seems to be a rather low notification threshold on a PC that has 3GB installed, not even 10% of RAM being used and that message is triggered.

That's a new feature in AVG, and I suspect it might need a little tweaking. Unfortunately AVG didn't seem to provide any user adjustments into that feature, so the best thing to do is turn it off if that warning message appears too often.

Here's an AVG support thread where the users says he gets that message with IE, Chrome, and Firefox.
http://forums.avg.com/us-en/avg-forums?sec=thread&act=show&id=180124

thanks - I went to the AVG forum that you suggested - your answer helped and I appreciated you taking the time.

You're welcome.