Update (2014-04-03): I got information from Sebastian the bugfix landed in 126.96.36.1999 version. Great job!
Update (2014-03-11): I exchanged a few e-mails with the author and the fix is on the way:
thanks for the details. I was able to track that issue down. When passing large data (like a 10 MB image encoded as data: URL in your case) to the background page, Safari freezes. However Adblock Plus can easily avoid this. Since we don’t block any non-HTTP requests, we can just bail out in the “beforeload” handler, when it is a data: URL, instead of sending the URL to the background page for further processing.
So, it looks like a bug in Safari itself. I will try to prepare a test case, then report a bug on bugreport.apple.com and update you with results.
Original post (2014-02-26):
Disclaimer: the purpose of this post, of course, is not to blame the author of this excellent add-on, but rather to prevent frustration of Safari users which may encounter this issue.
Initially I had planned to describe the whole investigation procedure here, but finally I decided to spare you the technical details. To make the long story short, here are the facts: Safari 7.0.1 with Adblock Plus for Safari (beta) installed may become unresponsive and freeze when uploading certain files. Precisely: not uploading, but rather when a webpage uses a FileReader API (eg. to generate a thumbnail of uploaded image). I discovered it by accident trying to recover my Battle.net account, but this is problem is not limited to this Blizzard service.
Important: I don’t know the exact criteria needed to be met for this bug to appear, but it doesn’t happen with just any file, however, it is reproducible in a 100% for certain files* (PNGs and JPGs) — kudos to Dariusz Siedlecki for helping me with testing/investigation.
I plan to notify the extension’s author soon, but until this problem is fixed, I recommend to temporarily disable Adblock Plus for a time of such upload:
* — if in doubt, have a look here, then click on Result and upload your file. If your Safari hangs on this site as well, you are affected.