Flash uses RGBA32 when wmode=transparent. We draw the flash content to a gfxImageSurface right now, which does not support RGBA32. As a result, transparent mode won't work and we force all flash instances to wmode=opaque (which uses RGB565). We need to support 32bit surfaces and make it work with Flash.
This one has been fixed for a while
(In reply to James Willcox (:snorp) (firstname.lastname@example.org) from comment #1)
> This one has been fixed for a while
does "a while" mean pre-aurora merge? if so, mark status-firefox11 to fixed