That sucks bro, I may have found a solution as I had a similar problem but not with GoG though.
Make sure that your not running any extensions for downloading content (i.e. your not using the default download manager.) You can disable these extensions and check the by clicking Tools-> Add-ons -> Extensions.
Before you try any of the below, simply open up your addons/plugins/extension manager windows and close them. Sometimes just doing this has been known to solve the problem. Hopefully something below will work if that didn't. If nothing else you have a good place to start :D
1. Close FireFox completely (start task manager and make sure that all FF processes are closed/stopped. Then delete the file named "compreg.dat" from your profile directory. This should be something like the following for XP
C:\Documents and Settings\<Windows login/user name>\Application Data\Mozilla\Firefox\Profiles\<profile folder>
2. I know this sounds kind of silly but make sure that if you are using something below FF3 (such as FF2) that your "history.dat" file isn't set to read only.
3. Once again make sure you have FF closed completely as outlined above, open your profile directory (as outlined above) and try deleting the "downloads.rdf" (this file is called "downloads.sqlite" in FF3 or later, however if you have both of these files delete them both.) As a note, you may have to set 'show hidden files and folders' on your explorer settings for your profile folder in Windows. This can be solved in the future by selecting "Tools -> Options " and unchecking "Remember what I've downloaded" Or alternatively you could go under "Privacy / Private Data" and select "Clear my private data when I close FireFox" then click the "Settings..." button and select "Download History" in the "Clear Private Data" window.
4. Choose a different download folder by typing in "about:config" in the address bar of FF find the option labeled "browser.download.lastDir", right-click on this, and select "Reset."
To choose a different folder go to "Tools -> Options" in the menu, under "Main / Downloads" select the "Save files to" option and click "Browse".
-- Total fail people.
LOL totally thought this was from this year.
Post edited May 27, 2011 by jurijchrul