I updated my firefox from 119.0.1 to 121.0 two days ago, and have noticed a for my usage quite significant change:
When I have a page, say a search engine query or a gallery of links on a page, and I open one then go back, previously I got the cached version. Within reason of the cache size I could go back a few pages even days later and critically see them as they where, just like I would expect for a tab I have open.

I use this behavior to work through essentially todo lists, so now that the lists get reloaded every time I visit them, this combines with server side caching to make the list jump around quite annoyingly.
My expected behavior would be the cached back history being served when available, except when I manually hit F5.

Was this change intentional? Is there any way to get the old behavior back?

Edit:
It seems to be a bug and only happen on some profiles, potentially dependent on some metric related to heavy use, like number of open tabs and windows.
Edit:
It seems to be related to uBlock Origin.
Edit:
It is definitely an issue within ubo, I will add a link to the issue there when I create it.
Edit:
It seems to be caused by the “AdGuard Tracking Protection” filter list within ubo.
Edit: issues:
ubo filters: https://github.com/uBlockOrigin/uAssets/issues/21841
AdguardFilters: https://github.com/AdguardTeam/AdguardFilters/issues/170172
Edit:
It was fixed a few minutes ago, the changes should percolate through to ubo soon™. Thx Yuki2718.