O modo privado não desativa as extensões
Você pode iniciar o Firefox da seguinte maneira:
firefox -private -safe-mode
-private
Opens Firefox in permanent private browsing mode. Firefox 3.6 and
later only.
-safe-mode
Launches the application with all extensions disabled, for that launch
only. (Extensions are not loaded, but are not permanently disabled in
the Extension Manager data source).
Fonte Opções de linha de comando
Modo de segurança
Safe Mode is a debugging startup mode available in Firefox,
Thunderbird, and SeaMonkey 2, where all added extensions are disabled,
the default theme is used, and default localstore settings (toolbar
settings and controls) are used.
Safe Mode also disables hardware
acceleration, the Just-in-time (JIT) JavaScript compiler, and any
changes made via userContent.css and userChrome.css. Any changes made
to preference settings remain in effect in Safe Mode, however, and all
available plugins are used. To summarize:
Safe Mode temporarily affects the following:
- All extensions are disabled.
- The default theme is used, without a persona.
- The Just-in-time (JIT) JavaScript compiler is disabled.
- The userChrome.css and userContent.css files are ignored.
- The default toolbar layout is used.
- Hardware acceleration is disabled.
Safe Mode has no effect on the following:
- The status of plugins is not affected.
- Custom preferences are not affected
...
For Firefox, one of these:
firefox -safe-mode
"C:\Program Files\Mozilla Firefox\firefox.exe" -safe-mode
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -safe-mode
Fonte Modo de segurança