Changes are now included in the current nightly build of Firefox 19.
Using NoScript 2.5.7 having ABE activated, internet access using manual proxy or PAC file is now possible again.
According to comment 34 the issue should be fixed in Firefox 19.
I just installed the actual Nightly version 19, but the problem is still existing.
Do you have to change something in addition within NoScript ?
Using Firefox 18 (Windows 7 x64) and Noscript 2.5.6, it's not longer possible to get internet access when using a proxy. After deactivation of Noscript, everything works fine. It also does not work with Noscript 2.5.7rc3.