Removing an extension should not have a huge impact on the RAM usage. I assume Outlook in the browser just requires a lot of RAM. Browsers and huge browser apps require ton of RAM, that sounds normal to me. Especially if you have lot of RAM, then Firefox will make use of that more, because a lot is available. That ensures a fast operation. Reducing the RAM usage might cause Firefox to cache data on the filesystem, then it will get slower, if it really needs that much RAM.
For testing purposes create a new fresh Firefox profile, which is basically what you get when you install Firefox new without your personal configuration and extensions. Try using Outlook in a fresh profile. Also try a different browser that is not based on Firefox (in example Brave) to see if it requires that much RAM too. This way we know if its because of the App, because of your profile or because of Firefox.
Still 5 years late. But better late than never. :D