Some of these have already been reported, but it may be useful to outline Gmail-specific problems.
(1) Signed messages sent from Gmail in plain text mode fail to verify in other mail clients and even in Gmail itself.
(2) Inline signed messages sent from other clients sometimes fail to verify in Gmail.
(3) Signed messages sent from Gmail in rich text mode verify in Gmail but not in other clients.
As previously reported, sometimes messages can be successfully verified in Gmail by using the "show original" feature and verifying that text.
If this doesn't work, you can cut and paste the "show original" text into gpg manually: under Windows, type gpg --verify, paste the text, type Ctrl-Z on a new line; in Linux, Ctrl-D instead of Ctrl-Z. Many more messages verify this way.
Finally, if gpg still fails to verify, you can paste the text into a file and manually join up any line that was wrapped. This takes care of everything else that will not verify and brings the success rate to 100%.
What I don't understand is why, sometimes, FireGPG will fail to verify a message, but if I cut and paste the text *from the regular display window* into gpg, it verifies fine.
Also, many mail clients do their wrapping before signing, but sometimes Gmail still distorts the text when displaying. I've noticed that even if nothing extra has been wrapped by Gmail during display, sometimes it will remove a leading space on a line. This causes the signature to fail.
For outgoing mail, the only way I can see to beat Gmail's wrapping is for FireGPG to wrap the text first, then sign it, then send it. Otherwise it won't verify in other mail clients. For incoming mail, is it possible to have FireGPG get the "show original" text instead of the text from the display?