I suspect that either Google has opted to block this in Firefox for Android, or it is somehow a security feature. I can't understand it, though. Scrolling screenshots can be performed in other apps.
I was able to do this on my Samsung flip 3 but now that I switched to a new Google Pixel it's not available for firefox. I'd rather not have to use Chrome in certain situations just to have this feature.
I have been using OnePlus 8t for 4 (after 10/2020) years and this feature has been working on their OxygenOS from the beginning and it is still working.
A couple of days ago I changed my daily driver to Google Pixel 9 Pro with Android 15 and now the rolling screenshot feature is no longer available with Firefox. I can use it with Chrome and with many more apps though.
When will this core feature be fixed? I am very disappointed that the feature is not working even many people have been complaining about the situation for years.