While this shouldn't affect most users (and by and large is probably a good thing), I noticed that Firefox 29 started automatically aborting requests after 300 seconds if the server has responded. As a developer, I often have test scripts that can take a very long time to run to completion, so this change was causing me some issues running a script to validate some data.
The change appears to be with the network.http.response.timeout setting in the about:config menu. It appears this setting used to be undefined, but it now set to 300.
It appears setting the value to 0 will reset the behavior so that Firefox will no longer automatically abort requests. Once I made this change, my test script starting running successfully (it's final runtime was about 6 minutes.)
Anyway, I don't expect this to affect most people. I think the impact is going to be felt mostly by developers who might have unit tests that run via a browser or other test scripts which could take a very long time to run.