Theoretically Tor Browser's fork is just another development branch of Firefox, just like Mozilla's primary development branch, and they are "uplifting/backporting" individual patches into stable the same way Mozilla might backport crucial dev-branch improvements/bugfixes into stable.

But they aren't landing patches on "stable", but on the upstream development branch (i.e. mozilla-inbound/central). Since it seems more reasonable to view tor browser as a release branch of firefox than as a development branch, it seems like they are using "uplift" in the opposite sense to the normal Mozilla usage i.e. they are taking a patch from a product branch and moving it to a development branch. In that context "upstream" might indeed be a less confusing choice of words.

On the other hand I think it's clear what they actually mean here, so probably not worth worry about too much.