Proton 4.3+, Catalyst 4.3+
Starting with Browsium Client 4.3 the Firefox extension has been updated to use the WebExtensions extension model. For users who wish to use the legacy extension (4.2.1) we have provided a command line parameter for the Client install package. You should only need to install the legacy Firefox extension if you are using Firefox versions 42 through 51. For Firefox versions 52 and newer we strongly suggest using the default Firefox extension based on Mozilla’s new WebExtensions model.
To install the legacy Firefox Browsium Client extension (4.2.1) you will need to install the client with the following parameter.
Note that the Proton functionality introduced in 4.3 (such as file upload detection, log-in detection) is not present in the legacy extension. If you are only using Catalyst you will not see any difference in functionality. If you wish to install the current Firefox Browsium Client extension at a later time you will need to uninstall the Browsium Client and reinstall it being sure to omit the USE_LEGACY_FIREFOX_EXTENSION parameter.