Hello, there are settings in Firefox that bypass any extension so you have to ensure they are correctly set, you can check https://protonvpn.com/support/browser-extensions#firefox

Here we give more details about the protection it offers: https://protonvpn.com/support/browser-extensions#faq-question-1677757419032

Note that we don't provide the extension as a complete replacement of the desktop application. Depending on your threat model, you may need one, the other or both.