So, you found out that there's a new firmware version available for your Canon camera. You went to the Canon website and found...that there are only Windows and Mac versions available! What now?
Just download the Windows version. The EXE is a self-extracting ZIP file. If you don't have WINE installed, Ubuntu (and probably other distributions) will open the file with the default archive-extraction utility. There should be a firmware file (extension .FIR) and some instructions in PDF format inside. I had a bit of trouble extracting them; it seems that the Ubuntu archive-extraction utility, file-roller, doesn't want to extract single files from self-extracting ZIP archives. Just extract everything and ignore what you don't need. Follow the instructions in the PDF file of the appropriate language, and you should be good to go!
In the future, it would be nice if Canon would just provide a plain old ZIP file. Every reasonably modern release of every common OS supports it.