i got a reply from blaagh on birdsite https://twitter.com/blaagh/status/1237056705931993088 explaining what happened:
https://pkg-status.freebsd.org/builds/default:quarterly:121amd64:527970:beefy2
[121amd64-quarterly-job-01] Extracting nspr-4.24: .......... done
===> firefox-74.0_1,1 depends on package: nspr>=4.25 - not found
Quarterly package build failed due to an out of date dependency :/
the #freebsd folks got on it, & both firefox and thunderbird should be back in the repo soon. thanks, everyone! https://twitter.com/woodsb02/status/1237314591262003201
i was pointed to a mailing list message which explains in detail what broke and how it was fixed: https://lists.freebsd.org/pipermail/freebsd-ports/2020-March/117932.html
now i just wait for a `pkg update && pkg search firefox` to produce the expected result, then i can unlock and upgrade.
thank you for the package locking feature, #freebsd. i mean that sincerely. with firefox locked against removal, i can at least keep working until this is sorted.