Firefox freezes temporarily when a download begins
I noticed this behavior after upgrading to 15.0.1. The browser window freezes for about 10 seconds or so after I've selected a location to place the file, and in that time period, the download window doesn't pop up. After that, all the browser's functions continue normally, and the download begins.
I normally download on Private Browsing mode, so there's no download history. This only seems to happen for the first download every time I start up my computer. Subsequent downloads happen normally and without any freezes.
EDIT: I have tried to use the Reset Firefox Feature as well, checked the solutions to download hangs and freezes on kb.mozillazine.org and attempted to apply the solutions. After all the above and everything short of a clean reinstall, the problem is still there.
The unresponsive script mentions something along the lines of downloadutil.jsm.
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I've attempted to troubleshoot this by downgrading Firefox to 15.0, but this doesn't solve the problem. The freezing still occurs.
I have also confirmed that this temporary freeze occurs every time I start up Firefox and attempt to download/save a file for the first time.
As some additional information, it appears to be something related to scripts. I was unable to get a screenshot of the notification, but a download script was mentioned to have failed or errored.
So after a few hours of troubleshooting, I've confirmed that the cause of these temporary freezes is Microsoft Security Essentials' Real-time Protection feature.
The freezes happen while it is switched on, but after switching it off, they are no longer present.
Is it possible for the development team to find a way around this, or is this something on Microsoft's end?
Does Microsoft Security Essentials have a cloud component where it checks URL or file reputation? If so, perhaps that service is experiencing some latency. Try turning that off, but leaving traditional real-time protection turned on.
You also can disable Firefox's integrated scanning feature, since this does not add much value when you already are using real-time scanning. This kicks in toward the end of the download process, so it might not be relevant, but you could try it anyway. To do that:
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful. This is Firefox's about:config preferences editor.
(2) In the filter box, type or paste scan and pause while the list is filtered
(3) Double-click browser.download.manager.scanWhenDone to switch it from true to false.
Not to my knowledge. The real time component is the only component that does real-time scanning, and the only component that can be toggled on or off in MSE's settings.
I've already switched off the post download scan via about:config, which has had no effect.
And almost as suddenly as this problem appeared, it's now gone. I added an exception to the folder I usually download to, and that seems to have solved it. But for some strange reason, even if I download to other folders, the freeze no longer occurs.
The only other cause I can think of is my own habit of removing all cookies from sites and clearing the cache even on Private Browsing mode just before I'm done browsing, just in case. I figured that might be it in my rush to keep restarting Firefox - and forgetting to clear the cache and cookies in the process - after which the problem disappeared.