Your problem is not really related to firefox. Your disk is just plain full. Also you cannot remove files through open dialog boxes. The add/remove buttons are just for bookmarking locations for faster access, the remove button does not actually remove files/directories. What distribution of linux are you using? If you have a gnome window manager (for example like ubuntu) you should have a bar on the top of the screen where is a menu item called "Places". This probably has the Downloads "shortcut" as well (since you mentioned you have it in the open file dialog box). Clicking on it should open the directory where the downloaded files are. Select the files you want to delete and press delete :). This will send the files to the trash. To remove them completly you still have to remove them from the trash. This can be done by clicking on the recyclebin icon in the lower right corner of your screen. In the trash select the files and press delete again. Hope this helps..