Comment 4 for bug 914312

I was just searching for this error and found your report. I've just being experimenting with turning off one add-on at a time and the bug seems to be from (Gnash) Shockwave Flash.
I too am using Gmail + FF + Ubuntu. I use many other add-ons but they don't seem to affect this bug.
If I disable Gnash's Shockwave Flash, the console errors stop. This is easiest to see by clearing the console each time, you probably know this doing development. If I re-enable Gnash and refresh GMail's page, the errors recur. Thanks to Firefox's way of handling add-ons, I didn't even need to quit FF and reload.
I'm using Gnash's "Shockwave Flash 10.1 r999. Gnash 0.8.10dev". Googling, I haven't find much about Gnash's latest version other than http://www.gnu.org/software/gnash/download.html
Now I'm off to dig for info on this add-on's bug. I don't fancy debugging Gnash's source code, so disabling it temporarily is a kind of workaround, much as I like Gnash because it's not Flash.