Hi Shade,
I'm using the current BETA 8 version of Firefox and haven't experienced any error message like you describe in your original post. Not even with version 3.6.10, 12, or 13.
The game loads fine with the BETA 8 version. Page load times are fast too across the web, but since BETA 8, flash crashes have been a little more frequent. The new BETA 8 has increased the off-line storage to a full Gig as well. I also like the bookmark button in the browser.
I downloaded BETA 7 several weeks back with some apprehension, but after using the browser, I found it to be quite stable for web browsing, and playing the game being it's a BETA browser. I'll continue to use it until the BETA 8 bugs start showing-up, and surprisingly haven't to date. I understand the full version of Firefox 4 will be released some time next month.