Possible to change/edit which letters are underlined in context-menus?

I want to change some of the underlined letters for quick-selecting context menu entries, particularly, for Firefox (but knowing how to do so for general Windows 7 context menus would be great, too).

For example, if I select some text on a page and right-click, the entry for searching Google has its first "S" underlined. I'm wondering where I might go to edit/change/add these.

Download the source code, modify the source, recompile, voila.

^^ that will work for FF and other open source programs.

There's not an easy way to do that in general. For some executables you could possibly modify the resource assets and keyboard accelerator table to change accelerator/shortcut keys, but good luck with that if you're unfamiliar with recognizing executable formats made by different compilers and/or need to make changes which increase the size of or shift the positions of those assets.

There is this, but I think it is currently broken by changes in FF 29 or 30.