No, that isn't possible. Those underlined letters are to show the access key for (context) menu items and items on built-in about pages (use the Alt+Shift modifier for the latter).

I believe that is possible with a file userContent.css. You have to create that file yourself and finding code may take time; I would try searching for it on the FirefoxCSS sub of Reddit.

I don't think this is possible via userContent.css/userChrome.css as the underline is likely added via JavaScript and is beyond control of those CSS files (you would have to replace the underlined version with the bare version).

It is possible to remove it goto about:config then in filter search for ui.key.menuAccessKey and change the value to 0 then restart the browser