different launchers for Firefox versions

Question

I'm trying to run a legacy copy of Firefox alongside the latest greatest Firefox. This works fine, I have two launchers like so in my .local/share/applications folder:

[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=Firefox 29
Icon=custom.png
Exec=/usr/local/firefox-29/firefox --no-remote -P "Firefox 29"
StartupNotify=false

and

[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=Firefox Trunk
Icon=firefox.png
Exec=/usr/local/firefox-trunk/firefox %u -P Trunk --no-remote
StartupNotify=false

It works, but after I start the two browsers they end up sharing a new icon.

I found a launcher directive called StartupWMClass, and tried making them not match each other... but... it turns out both apps have the exact same WMClass:

$ xprop WM_CLASS                         # then click on FF 29
WM_CLASS(STRING) = "Navigator", "Firefox"
$ xprop WM_CLASS                         # then click on FF trunk
WM_CLASS(STRING) = "Navigator", "Firefox"

Is there a way to specify the WMClass for an application?

Answer 1

You can't specify the WMClass for Firefox 29, but you can for newer versions using the --class=Something command line argument.

So I adjusted the firefox trunk launcher like so:

[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=Firefox Trunk
Icon=firefox.png
Exec=/usr/local/firefox-trunk/firefox %u -P Trunk --no-remote --class=FirefoxTrunk
StartupNotify=false
StartupWMClass=FirefoxTrunk

... and added StartupWMClass=Firefox for the Firefox 29 launcher, and now when I run xprop WM_CLASS...

$ xprop WM_CLASS                         # then click on FF 29
WM_CLASS(STRING) = "Navigator", "Firefox"
$ xprop WM_CLASS                         # then click on FF trunk
WM_CLASS(STRING) = "Navigator", "FirefoxTrunk"

Yay!

So now the launchers each work as expected, with their windows attached to the correct launcher, and no new phantom launcher gets created.