Try in Firefox's safe mode

• do not make changes when you see the first options window - click on continue
• use the default themes
• if the above does not help then whilst still in safe mode disable all plugins
• see also troubleshooting extensions and themes and troubleshooting plugins

What is it that you are trying to do download files from somewhere on the internet to your own computer ?

• if so see also downloads Window

I am already running the default theme and now I have tried to run Firefox in safe mode with all plugins disabled, but I still can't resize the builtin File upload dialog. If I don't remeber completly wrong it was rezizeable in earlier version of Firefox.

It is not the download window that appears when you download a file from Internet to your computer, it is the upload window that shows when you want to upload a file from your computer to a server.

Now when I wanted to upload the image of the upload window, I found that this upload window is resizeable, it must be site specific?

You could try the advanced javascript properties

Firefox button -> Options -> |Content | -> (Javascript -> Advanced)

I didn't change anything, I tried to disable all javascript options and then I tried to enable all of them, but no change.

But I found that the difference between the file upload below (Add images) that I can resize and the one that I have a problem with, is the 'multiple' attribute. I removed it with the help of Firebug and then the File upload window became resizeable! I don't know if it is a bug or not, but at least I know why it doesn't work on that particular page.

HTML: <input type="file" multiple="multiple" id="torrent_upload_file" name="torrent_files[]">

It sounds like whatever it is, is as you originally said site specific.

Yeah, but it used to work with that page before version 7 (if I remember it right) and multiple is a valid HTML5 attribute. I will see if I can find out how to report a bug.