firefox/qt: added dark mode troubleshooting

wiki/firefox.md

@@ -320,3 +320,17 @@ security.family_safety.mode = 0
 
 Additionally check [the telemetry section](#telemetry) and
 [the sponsoring section](#sponsorings).
+
+## Troubleshooting
+
+This section will focus on errors and the fixing of errors of Firefox.
+
+### Dark Mode Not Used For File Picker
+
+It is possible and a known problem on [Linux-based systems](/wiki/linux.md) that Firefox may not
+recognize the system dark mode correctly.
+This can result in a light themed file picker despite all other programs on the system like
+[GTK](/wiki/linux/gtk.md) or [Qt](/wiki/linux/qt.md) using the dark mode already.
+To fix this simply navigate to `about:config` and - if not already existing - create the value for
+`ui.systemUsesDarkTheme` and set it to `1`.
+This should immediately and without rebooting switch the file picker to use dark mode.

wiki/linux/qt.md

@@ -44,3 +44,16 @@ After the installation it can be selected by `qt6ct` as described in the
 
 For setting the same theme for GTK refer to the
 [corresponding entry](/wiki/linux/gtk.md#achieving-a-system-wide-uniform-look).
+
+## Troubleshooting 
+
+This section will focus on errors and the fixing of errors of Qt.
+
+### Incorrect Configuration: `The libqt6ct.so plugin is compiled against incompatible Qt version`
+
+When updating the system it is possible that the theme will not work correctly anymore.
+Often the error `The libqt6ct.so plugin is compiled against incompatible Qt version` is responsible
+for this.
+To fix it most of the time it is enough to recompile the `qt6ct` package.
+Afterwards a reboot might be needed for the theme to be displayed correctly again.
+