[Osiris]

When you click on a file Firefox determines if there is a plugin that can handle the file type or if there is a so called download action defined. Download actions are user defined actions that come into effect when the box “Do this automatically for files like this from now on” is checked in Firefox during a file download.
Checking the box has the effect that Firefox does not ask what to do with this file type from then on but simply performs the default operation selected by the user which means either saving or opening the file automatically with the selected application.
This can lead to some complications later on when another application should handle the file types for instance. There are two possibilities on how to handle such a situation. The first is to click on Tools > Options to open the Firefox configuration. The download actions are available under the Content tab when clicking on Manage next to the entry File Types.
Single download actions can be removed there. The other option would be to reset all Download Actions if things have been really messed up and you want to start over. The steps are the following:

• enter about:config in the address bar and hit enter
• right-click the preference browser.download.pluginOverrideTypes and select reset
• right-click the preference plugin.disable_full_page_plugin_for_types and select reset
• locate the file mimeTypes.rdf in the profile folder and delete it. Make sure it is in the profile folder and not in the programs folder

Properly Reset Firefox Download Actions