I've had exactly the same problem -- I'm on a Mac and, after upgrading from 2.51 to 2.6 I could upload but no longer insert pics, no matter which browser (I-Cab, Safari or Firefox) I used. The browser pop-up uploader would work right up to the point I clicked "Insert Pic" at which point the pop-up would go blank. Following forum tips, deleting pics, disabling all browser filters, completely reinstalling the browsers, completely reinstalling WordPress, etc, made no difference. For clarity, I omit several days of stupefied disbelief and hitting things.
Astoundingly, the answer to my problem turned out to be capital letters. For style purposes, I had the site name in Dashboard > Settings > WordPress Address (URL) as ThisIsTheSiteName.com, which had previously caused exactly no problems at all. Changing this to thisisthesitename.com fixed it.
Might I politely suggest that the 2.6 pop-up uploader be twiddled to flatten any URLs, or at least to feature a warning rather than going blank and hanging? Thanks.
(NB. Also posted in http://wordpress.org/support/topic/189738 , just in case.)