Any idea how to get these working? I like to use the thumb buttons on my mouse for things such as reloading. The game has entries for mouse 4 & 5 and you can make the assignment, but pressing these buttons does nothing...

Yeah, it's strange. This is an SDL2 game, so I checked with a test program and my side buttons are indeed reported as button 4 and 5 by SDL2 (they're 8 and 9 in xev, so I needed to check what SDL2 thinks of those)

But the game does something strange, it maps buttons 1-2-3 as 0-2-1, wheel up/down as 4-5, and what is available in the menu as mouse4/5, is mouse3/6 in the config (~/.config/fury/fury.cfg) and as you know, doesn't work at all.

I've tried setting MouseButton7/8/9 in the config (since those settings are there, just not presented in the menu), but that doesn't work either.

I guess if you really need to bind those, you can run xbindkeys to remap b:8 and b:9 to some keyboard buttons (game has decent keyboard binding menu, but mouse button remapping is idiotic, so just remap to some keys to make life easier), or bug Voidpoint :)