Csdrhrt

On Chrome browser, you can select which extensions are active in private mode (incognito). How to do the same in Firefox please? (By default Firefox allows all add-ons in Private mode).

Add-ons

At least through version 63, Firefox lacks a native feature to selectively control which add-ons are allowed in private mode; the enabled/disabled status of each is the same in private mode (although all add-ons are required to comply with the private mode rules regarding data retention). But there's a workaround to accomplish what you want.

You can have more than one profile, and each profile is independent as far as what add-ons are installed or enabled.

• Create a new profile, including just the add-ons you want in private mode. If you're in Firefox, enter about:profiles in the URL window, and that has an option to create a new one.


• 
  

  Create a shortcut to launch Firefox in private mode using that profile. The command line would look like:

  
  
  

  firefox -private -P "new_profile_name"

  
  

Bookmarks and Other Profile Content

As you note in a comment, this still isn't exactly the same as the Chrome feature because Chrome retains other settings and information, like bookmarks, in incognito mode. So you need a second workaround if you want that similarity.

You can do that with controlled syncing. Firefox has a feature that allows you to selectively synchronize most of what reflects "you": preferences, add-ons, log-ins, history, bookmarks, etc. It can be accessed through Preferences | Firefox Account.

You obviously don't want to synchronize some of those options with a private mode profile. For example, add-ons (since you want them to be different). You also probably don't want to sync history (Firefox won't retain the history when you finish the session, but I'm not sure whether it would sync the visited sites during the session, and that would defeat the purpose of private mode). Login information is another thing private mode erases at the end of the session, so you probably don't want to sync that.

In any profile, the sync selections control what that profile shares with, and updates from, your other profiles. So you can have several computers that share everything, and a private mode profile that only shares and updates certain options.

Syncing uses an email address as a unique identifier. You create an account tied to an email address. Firefox will remember the account ID for you and automatically sync when you start or there is something to update (or you can activate and deactivate syncing manually).

If you will want a lot of add-ons in private mode, you might want to initially sync them just to install all of your current add-ons in the private mode profile. Then deselect add-ons for syncing, and remove or disable the ones you don't want active in private mode.

Profiles vs. Portable Firefox

Firefox, itself, is like a browsing "engine". Everything you do that makes it different for you is stored in your profile, which is located separate from the Firefox application software. That includes preferences you set, add-ons, the history of sites you visit, your bookmarks, etc. When you update Firefox, it updates the software, but doesn't touch your profile (well, except to verify that all of your add-ons are still compatible).

This also allows things like having multiple users on one computer, or a user with multiple profiles, while requiring only one copy of the Firefox software to be taking up space on your drive. Your "user experience" comes from the Firefox application using your profile.

Downloading a separate portable Firefox, as you suggest in a comment, would produce sort of a similar result, but it's not the same. That would add another copy of the Firefox application just to get another profile. There are also some subtle differences in the portable version, like it can have only a single profile.