Hi trutty,
The ‘unable to store in cache’ error is new to me. I’m guessing here a little but is firefox failing to write to it’s temp folder in AppData/Local maybe? might be worth clearing firefox’s cache manually, I think it can be done via the Options->Advanced menu.
Also, is there any difference between development builds and shipping builds? The out of memory could just be that your game was very close to the limit in 4.6 and new code in 4.7 pushed it over the edge. The fact the 64bit browser works suggests this to me, if shipping works then again would suggest that’s the case.