Is it possible to change the bindings for
mouse5 (the thumb buttons) in Netbeans? I can't find any option in the NB keymapper, nor can I find anything on Google!
I'm using Netbeans v7.0.1.
No you can't. But you can vote for this bug. You'll have to create an account for the netbeans site if don't allready have one.
What I've done to accomplish the desired result is bound my mouse buttons to keystroke combinations and then map those combinations in Netbeans.
Depending on your mouse manufacturer but it really helps if it already comes with a software to allow overall and per-program custom button bindings (e.g. in my case SetPoint by Logitech made it an ease - with a caveat of the middle button for which it is unable to support per-program binding for my M510).
If such manufacturer provided software isn't an option for your mouse you could accomplish the same result with AutoHotkey and the Remapping Keys and Buttons docs. e.g.
The Remarks section also covers how to make a binding for a specific window so you could have more freedom in what and how you decide to map.