It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files by Firefox.
You can try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See http://kb.mozillazine.org/Unable_to_save_or_download_files

Thanks Cor-el,

I already tried to disable the antivirus (Bitdefender 2010), but it did not solve the problem with 3.6.(4-10). For 3.6.3 no need to even try because everything is OK.

Thanks anyway

I think I'm having this same problem too, unless I'm misunderstanding your problem. When I try to do something like right click an image and save it, Firefox crashes. I tested this with a couple of different images and image types and it crashed every time. It would also crash again once more upon reopening Firefox, though it would work the second time around of opening it. I tried disabling some add-ons as well and it didn't change anything.

Is this the same problem? I'll make my own support thread if it's not.

I'm using Windows Vista and I know it's nothing to do with my anti-virus. I let Firefox auto-update and I'm on 3.6.10.

To Danillama,

It does not seem to be the same problem.

My problem is actually to "save as" a page. "Save as" is also triggered by a download operation. My problem occur with ANY page, and ANY.