Hello @rohit_chandna,

Firefox uses addons.mozilla.org and and Edge uses microsoftedge.microsoft.com/addons for adding new extensions.  So you can create a URL list that blocks those two and then block that URL list in a custom category.

This will block new plugins from being added via the stores for the respective browsers.  However, if the user already has the extension installed then it would not block the existing one.  However since Netskope would intercept the outbound traffic from the plugins, you can block that traffic based on category, activity, etc.   In my opinion,  a better way to manage extensions is provided by both browser vendors (and Chrome) which is to use GPO or MDMs to control if and what extensions are allowed:

Edge: https://learn.microsoft.com/en-us/deployedge/microsoft-edge-manage-extensions-policies
FireFox: https://mozilla.github.io/policy-templates/
Chrome: https://support.google.com/chrome/a/answer/7532015?hl=en