68 people have same problem and 5 new this week. doesn't this urge you to fix the problem. I contacted fileserve many times but they have not found an issue. I strongly believe the issue is at fileserve. Internet download manager support team has some workaround but this is troublesome: click on the link -> click on slower download -> type the captcha -> press and hold the alt key. don't release. while pressing alt key click on catpcha confirmation button and wait the seconds to complete and green slower download button appear. then you can release the alt key. then press insert key and press "slower download".

this is troublesome. can firefox team make a workaround for it? I am still waiting you for a few months.

I didn't had to use any ALT keys and I don't use any Download Manager but the link you provided worked fine for me ... here is my suggestion:

-> go Tools Menu -> Clear Recent History -> Time range to clear: select EVERYTHING -> click Details (small arrow) button -> place Checkmarks on Cookies & Cache -> click "Clear Now"

-> Restart Firefox

Check and tell is its working.

You can also try this link:

Server not found

https://support.mozilla.com/en-US/kb/Server%20not%20found

of course I cleaned the all private data in all time range at the first. even did this too many times. alt key is shortcut key for internet download manager that prevents files getting downloaded by internet download manager and let the browser display the file (if it is file such as mp3 then will be played or if it is rar file then will be shown download dialog). insert key forces download by internet download manager. Indeed the error page is full fileserve page which have the "page you requested is not found" message in the center. Again it is "full fileserve page" with all the menus pictures etc so therefore it is not server not found issue. (I read your article.)

internet download manager support team told me that fileserve is using something (maybe page or file) that can be requested only once but not twice.

I think it is requested twice then the error message will appear. Perhaps first request by firefox browser and second request by idm. therefore preventing second request by alt key.