Firefox 3 Thumb Button brings up Save As dialog

This has been driving me nuts since I have upgraded to Firefox 3.0.1 on my laptop like a month ago. I’m still running Gutsy on that thing, and only the beta version (which crashes a lot) is in the repositories. So I pretty much did the same thing as back when FF 2.0 came out and I was running dapper. I snagged the statically linked linux binary from the website, dumped it into /opt/firefox and un-installed the old version. It works fine with a single exception – my thumb button was acting weird as hell.

This requires some explaining. I’m using Logitech VX Revolution wireless mouse and the btnx to detect and remap all the additional buttons on the mouse. I configured it so that the back and forward thumb buttons are set to emulate the Alt+ left arrow key and Alt+ right arrow key. These are of course Firefox shortcuts for Back and Forward controls.

To tell you the truth, I actually forgot how to press the back and forward buttons on the FF chrome. I just never do it. At home I am using MS Sidewinder mouse which also has convenient thumb buttons. I always use them while browsing because it is just so much faster than anything else. Faster than keyboard shortcuts because when I’m browsing one of my hands is usually holding the mouse. So thumb buttons are perfect for quick flicking back and forward between pages and I miss them when I’m forced to use a mouse which doesn’t have them.

Ever since I started using FF 3.0.1 on my laptop the back thumb button started doing something weird. In addition to sending the browser the “Back” signal it would also invoke the “Save As” dialog. Yes, it would just pop up in the middle of the screen. Attempting to use the dialog would be futile however. I tried to actually save whatever it was asking me to save several times, but Firefox never actually produced any files as a result of using this particular dialog.

It was nothing more than a constant annoyance. Every time I tried to go back a page, I would have to cancel this dialog. It was actually more annoying than clicking the buttons manually, or taking my hand off the mouse do do the Alt+ Left Arrow thing. Since then I have reconfigured btnx dozens of times, and kept searching Google for a possible solution. No luck. Eventually I figured out this had to be some Firefox setting and started digging in about:config.

Through bunch of trial and error tests, and good amount of luck I finally managed to identify the culprit. If you have this problem, all you have to do is to set:

middlemouse.contentLoadURL = false

From mozilla knowledge base:

Background:

This preference determines how to handle middle clicks in the content area. It was split off from middlemouse.paste, which now handles middle clicks in text fields only.

Possible values and their effects:

True: Load the contents of the clipboard as a URL. (Default for Linux/Unix.)

False: Handle middle-clicks normally. (Default for all but Linux/Unix.)

I actually never, ever use this feature because 90% of the time the thing in my clipboard is not a loadable URL. Besides, since I switch between platforms a lot, I generally don’t get used to platform specific features like that.

I have no clue why this particular setting kept picking up the Back Thumb button as some sort of “Save As” invocation. Note that this was not happening in FF 2.x, so go figure. Setting it to false solved the issue for me. I can browse like a normal person again.

Hopefully this post will help some other poor soul frustrated by this issue. :)