Why are the browsers only downloading some files but not others

My 12 month-old MacBook Pro just suddenly seems to not want to download certain files.

This sounds like a silly noob PEBKAC but it's not! For the past few weeks, most files that I click on in my browser which are not viewable in the browser (DMG, ZIP, etc) do not prompt me to save them. Instead of the Open with/Save File dialog appearing I just see a blank page and get the spinning wheel in that browser's tab. The status bar at the bottom reports "Transferring data from " but the download never completes, no matter how long I wait.

I've done some initial troubleshooting. Here's what I've ascertained:

1. I can use my web browser to view HTML pages, PNG images, JPG images, Flash sites, and even sites that load Java applets.
2. The issue is not specific to a file type (extension). I can download some ZIP files, but not others (see below).
3. The issue is not due to any ISP level filtering. My wife's computer, on the same network (older version of OS X) does not have this problem.
4. The issue is not related to the size of the file. See examples below.
5. From doing some digging on the mozillazine knowledgebase, I thought perhaps that my problem was a corrupt localstore.rdf file. I deleted this, both manually and using the "Safe mode" option, but it didn't seem to make any difference.
6. I've also tried resetting options related to browser.download in about:config, but this hasn't made any difference either.
7. It was at this stage that I tried other browsers, and came to the conclusion that it was something at the OS level. Yes, this problem occurs in ALL browsers that I have installed (Firefox 3.6, Opera, Safari and Chrome).
8. The issue happens regardless whether I'm connected to my home wi-fi network or a different wi-fi network (e.g. at work).
9. I currently have my firewall disabled in System Preferences.
10. (edit) After posting this question, I installed a download manager called Folx. Entering the URL of a file directly into this app seems to download any file just fine. However I'm stunned why the default browsers are behaving like this, and would prefer the lightweight browser default to a heavy, feature-rich third party download manager.

Here's an example of a file that I can download just fine. The Open with/Save file dialog appears, and I can specify a location to save the file. The file saves without problem.

• This freeware font:
  www.fontspace.com/livin-hell/carnivalee-freakshow
  (click the Download button — I didn't link to
  the zip file directly because the
  site checks the referer to discourage
  hotlinking)

And here are some files that display "Transferring data from somesite.com…" and a spinning wheel but never actually save any file to my local machine:

• This freeware DMG file: mac.eltima.com/download/downloader_mac.dmg
• This 512KB test.zip file on Dropbox: dl.dropbox.com/u/477850/StackOverflow/test.zip
• This freeware font: www.1001freefonts.com/font/BaroqueScript.zip

Now I'm stuck. Any ideas from the community are very welcome! Also apologies for removing hyperlinks that were added for me by a mod — I couldn't update the post without removing them 🙁