How to re-display the startup screen immediately after the Firefox 100 update

Firefox has reached version 100. Congratulations! So, this screen appeared upon restarting after the upgrade.

As I was looking at it, it disappeared. I wanted to see it again, but it wouldn't show up anymore.

I thought it might appear if I launched it using Firefox -P and specified a new profile, but a different screen came up.

I assumed it was tracking the state via some about:config setting, so I went through some trial and error to see how I could get the screen to appear again.

The second one was simple. It appeared after deleting the following two settings and restarting:

browser.startup.homepage_override.buildID
browser.startup.homepage_override.mstone

The first dialog-like one is a bit more involved:

browser.startup.homepage_override.buildID 20220422192727
browser.startup.homepage_override.mstone 99.0
browser.startup.upgradeDialog.version 99

Setting it like this and restarting made it appear. It seems like you just need to make it look like you were using a version prior to 100.

If you decide to try it, please do so at your own risk.