Interesting... Yes, I also see the issue in Firefox. Please correct me if I'm wrong, but is this not a case of Google not supporting Firefox? I also see the option there in Edge(built on Chromium). Someone more web savvy will have to answer.

Yes initially I posted this same to Google community, but this is their reply.

"Hey Christina, This is Google Chrome Help Community.

I don't think the issue is with Google Meet as it works fine on Chrome. So, it's more like a Firefox issue, you'd better reach out Firefox Support.

Greetings :)"

I too believe that making this feature supporting firefox can be carried out by google. But Not sure.

They said they "think" which means they are not sure. Someone closer to the Dev Team will need to answer. Like that person from the Chrome community, I am from the Firefox community. ;-))

I also found this...

https://support.google.com/meet/thread/75225531?hl=en

Also, just found this...

Why don’t I have the Change background option?

If you can’t view the Background blur option, you may not meet the following requirements:

   A supported browser version:
       Chrome version M84 or above on Windows, Mac or Linux. Learn how to check your browser version.
       Version M86 or above on Chrome OS. Learn how to upgrade your Chromebook.
       32-bit Chrome on Windows is not supported
   Chrome Hardware acceleration must be enabled.
   Browser must support WebGL 2.0. 

Looks like a Google not supporting Firefox issue to me because Firefox does support WebGL 2.0.

Thanks for the response :)

Interesting. As of last week, 12 May 2021, I had used background images with Google Meet in a Firefox browser. However, after yesterday's Windows 10 update (2021-05-18) Firefox is no longer compatible (again). Clicking on the "supported browser" link in the error message reveals a reason is basically identical to what jonzn4SUSE posted on 1/7/21 at 3:43 AM.

"Why can't I use the Change background option?

If you can’t use the Change background option, you may not meet the following requirements:

   A browser version that supports Change background:
       Chrome version M84 or above on Windows, Mac or Linux. Learn how to check your browser version.
       Version M86 or above on Chrome OS. Learn how to upgrade your Chromebook.
       Edge (Chromium) 85 or above on Windows or Mac.
   Chrome with Hardware acceleration turned on.
   A Browser that supports WebGL 2.0. 
   A 64-bit operation system. 

Enable Hardware acceleration

Turn on hardware acceleration on Windows or Mac:

   On the top right of a Chrome window, click Menu More and then Click Settings. 
   On the left, next to Advanced, click Down Down arrow.
   Click System Wrench.
   Turn on Use hardware acceleration when available.
   Restart Chrome.

Check WebGL 2.0 support

Check if your browser supports WebGL 2.0 at webglreport.com and verify that “Major Performance Caveat” is marked as “No”. Your browser might disable WebGL if the device is unstable or cause crashes. You can’t enforce WebGL on unstable WebGL devices. In some cases, it may help to update graphic card drivers or restart your browser. Learn more about WebGL support."