I'm reviving this issue because a previous unresolved thread on it was closed, and I have a solution that works for me.
The issue is in Yahoo mail. When composing a message in full HTML mode, the font size increases with each carriage return (new line or "Enter").
(This is still a problem as of June 2013 and the current version of Firefox for Mac - version 21.0)
The solution that works for me is:
View > Zoom > uncheck Zoom Text Only
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It should also work if you aren't zooming the page at all (View > Zoom > Reset) as that seems to cause it.
View-Zoom-Reset did not solve the problem for me... neither did changing the email format to plain text....
When I switch to Google Chrome the problem goes away...
Must I leave Firefox forever?!
Nothing in the Zoom land makes any difference for fixing the problem.
I found a different solution because Ctrl-0 (Reset) did not solve this problem for me, nor did creating an exception for yahoo mail. (I have only encountered the problem on yahoo mail.)
This is what worked for me: Under View/Zoom/Zoom Settings (No Squint)/Zooming, I had full-page and text-only zoom levels both set to 120%. Resetting the text-only to 100% solved the problem. (I then increased the full-page setting to 125% and the fix still works.)