Firefox: Disable delay on download dialog buttons

Try xdotool or IronAHK. I haven't used either of them, but they're basically tools for simulating mouse and keyboard events, which sounds like one way of solving your problem.

Edit: See the comments for the exact answer.

1. Tools > Options > Applications
2. Go to the line of the Content Type of whatever file you no longer want to bother with
   • If you want all future text files to just download, look for the line that says Text Document
   • For PDFs, look for Adobe Acrobat Document, etc.
3. Set the action to Save File
   • Or just set all file and content types as Save File and be done with it.
4. Click OK

Now, whenever you hit a file that is on this list and set as Save File, it will just download in the background. When it's done it will show a little pop-up in the lower right corner that the thing has completed its train ride to your computer.

If you come across a file type that was not in the Applications list section previously, you will still be prompted.

In which case you should:

1. Select Save File
2. Check Do this automatically for files like this from now on
   • In some cases, this may be greyed out
3. Click OK

For future files of the same type, go back to the first set of instructions and make sure it's set.

And if you don't want to see that dialog window pop up showing the progress of file downloads:

1. Tools > Options > General
2. In the Downloads section, uncheck Show the Downloads window when downloading a file
3. Click OK

No need to download or slap on any extensions.