Look into Changing download actions

1. Click the Menu button and choose Options. (top-right 3 lines)
2. Select the Applications panel.
3. The Applications panel will display. Select the type of file for which you want to change the default action.
4. Click on the down arrow under Action to select what you'd like to do when you click on a file.

Hope it helps.

To save time, the browser may start the download once selected. Than the browser uses the file the way you choose. (save / view)

Thanks. But I need to check the file SIZE before open/save it. If it is too large I need to cancel it. But while showing the Open/Save dialog box, the firefox downloads it silently and eats my data. If I select a application to open with, it downloads without asking.

I use a download manager that does show me the file size.

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Generally the browser don't behave like that. YOu need to check the setting first. Go to settings and select Always ask where to save file , then try to download the file , if problem persist, please feel free to let us know.