Note: I switched over to Linux (Mint) several months ago. During this time, Firefox has seen many updates. Before LinuxMint, I was on Ubuntu (14.04) and had the same issue - as follows:
Unable to set external source editor through gui:config. It seems to save "view_source_editor_external" to 'true' (this is good). The 'view_source_editor_path' does *not* seem to get saved in 'about:config'.
Was not an issue under Win7 (last year, have not been on a Windows System in over 9 months) and earlier versions of Firefox last year. The issue might be related to newer versions of Firefox under Ubuntu and similar. Just want to point this out, it's a great extension otherwise.