On a whim, I disabled three plug-ins: FlashGot, Link Extend and Download Manager Tweak. I'm not sure which one had been causing the problem, but it seems to have gone away. My feeling is that the problem was with Download Manager Tweak. Firefox seems to be running much faster and, with the download manager's window closed, I have experienced no further freeze-ups. At least not in the last hour. I'll post again in this thread if the problem returns.

It's Download Manager Tweak. I was getting this problem as of 2012-04-14, and do not have either of the other two addons that a_grush suspected.

For me, the problem was *mostly* worked around by having DMT put the downloads tool in a tab instead of its own window or the sidebar. I even made it a permanent app tab. The "error" message still popped up occasionally, especially when switching to the Downloads tab.

I *somewhat* worked around the bug when I experimented by having the downloads tool in the sidebar. The bogus error did still pop up fairly often in that arrangement, especially if you try to interact with the sidebar, e.g. re-loading a failed transfer, but much less frequently then when it was in it's own window, which caused near-constant "errors". That said, the sidebar function is essentially useless anyway, because you can't see the full filename, nor any of the useful buttons, like the reload-this-download button.

To get rid of the "error" message entirely, get rid of it DMT entirely.

To me, even the infrequent dialogs popped up in tab mode were too much, and I disinstalled DMT.

ThankYou!

This solution did not help me, because I am not running Firefox with Download Manager Tweak. Downloads continue to be problematic, stalling Firefox for long intervals. I have taken to using IE (UGH!) for my downloads these days because I cannot stand to waste my time waiting for FF to unconstipate itself! Sometimes a dialog box pops up telling me there is an unresponsive Java script. Sometimes not.

Sounds like another add-on that messes with the Download Manager. Try removing all add-ons that have anything to do with downloading (if that works, you can add them back one at a time until you figure out which one is causing the problem). If that failed, try removing all add-ons, and only adding them one at a time. Last, try ditching the entire profile folder; if that works, you can selectively add stuff back in like your original preferences file. If the problem comes back you may have to abandon everything from the original profile and re-install Firefox. Some Firefox problems can be very stubborn this way.

My guess is that some add-on you have, or that you tried out for a while, changed one of the geeky settings accessible through "about:config". Without knowing which one it, would be hard to track down. If you want to track it down, I'd start by examining the code of DMT and seeing what settings it makes changes to. DMT definitely causes this problem, even if not exclusively. I can make the problem appear and disappear at will simply by installing and uninstalling DMT. (In fairness to DMT, I run other add-ons, too, and it may well be an interaction issue between DMT and something else, otherwise more users of DMT would have reported the problem and it would have been fixed a long time ago.)

I am not now using and I have NEVER used DMT. Why bring it into the discussion?

Read what I wrote, please.  :-) Recap: We know that DMT will cause this problem. Something you did somewhere along the line also caused this problem for you and your browser. One (geeky) way to track it down would be a see what DMT does in its code to change about:config details, and then see if similar changes have been made, by you manually or by some other add-on you use or once used. Or just follow the simpler steps I suggested, if you are not technically minded and/or don't have time to kill on a detailed about.config bug hunt, which might get tedious.

I am technically minded, but I do not have the patience to wade through all the about:config details, or, worse yet, to read the program code in DMT itself. For this problem, I am pretending to be a mindless and clueless end-user of FF. Life is too short for me to do otherwise... Ben

The download manager displays the list of downloads in a richlistbox. If displaying that list takes a long time then newer versions of Firefox display this error. That is why richlistbox.xml is pointed at as being the problem. Download Manager Tweak (DMT) adds some features to make the richlistbox "richer" and can thus slow down the processing of the list. DMT doesn't create the problem, but can make it worse. To make this better you can:

1. Reduce the number of downloads in the list (DMT can actually help here, as it has an option to automatically limit the number of downloads in the list).
2. Get a faster PC or clean up the viruses that are slowing it down (only partly kidding :) ).
3. Disable the extra buttons DMT adds (Under options-> buttons, disable the Open Folder, Download, and Delete File buttons in the lower half. Displaying these requires checking to see if the downloaded file exists and the folder it was put into exists. That is probably the part of DMT that slows the richlistbox the most).
4. Disable DMT (I don't make any money off it, so it's your choice).

I'm the current maintainer of DMT, which is why I'm providing so many details related to DMT. I've never seen this problem myself and I have a five year old PC (soon to be upgraded) with several dozen files in the download list.

That all sounds good. The only suggestion that helps is to delete downloaded files from the list displayed when a download takes place.

Once again, I DO NOT USE DMT. NEVER HAVE! So it is pretty hard to disable something that is not installed.

The system has a quad-core Xeon X3220, way fast enough for everything else, and the system remains virus-free. It is simply that when FF hangs during downloads whining about an unresponsive JavaScript, it is very annoying.

So I won't use FF to download anything any more if clearing the list of downloaded files does not make it more responsive... Ben

@Ben: Again, try the other ideas I suggested if you find that keeping the DL list shorter isn't helping. IF that's the case, some other add-on must be adding stuff to the window to increase its processing time. No one 's saying DMT did it, only that what DMT does has this effect, so anything else that does similar things will also have that effect.

@squashr1979: That's unfortunate, since the main reason to use DMT is those extra buttons, and deleting stuff from the DL list immediately upon DL completion defeats a lot of DMT's purpose, like the reload button, etc. Like Ben, I'm on a honkin' fast machine. I'll try experimenting with DMT's ability to limit the number of items in the list, when I get a chance; I'd like it if DMT was usable on my system. PS: It would still be our choice even if you did make money on it. >;-)

The buttons I am talking about disabling are the ones inside the list, one per download. The row of buttons at the top can stay, so you can still delete or reload. The top buttons only test the one selected file, not the entire list. Also, I'm not saying removing from the list upon completion, but the option on the first option tab to limit the number of downloads in the list to some number.

Don't forget that I'm guessing on what is happening as I've never seen it.

Understood. I've reinstalled it, set it to not use the per-item buttons, limited to 100 downloads and 5 days, open in a tab (and stay open - I actually have it pinned), theme changes (whatever they are) disabled. Let's see how that works. NB: It strikes me that some of this stuff should be set this way by default, especially sane defaults for number of DLs and time to keep, and having the the per-item buttons in the list not only off by default, but set up with a warning if the aforementioned values aren't sane.

Life is lots better here now. Why? Because I deleted all the files from the list shown in the download window. There were maybe 30 or 40 of them, which, on the surface, would lead one to think that they would not absolutely KILL download performance. I did two large ISO downloads afterward and they went without a hitch. As far as I am concerned, there is no excuse for a download list to kill FF download performance. Although I seem to have found a workaround, this is a serious design flaw. I'll now continue my practice of using FF for downloads and other things too... Ben