https://hg.mozilla.org/releases/mozilla-release/shortlog/35f...
Basically, Firefox 98.0.1 is a point release only concerned with removing Yandex search for users in Belarus, Kazakhstan, Russia, and Turkey, and switching them all to Google by default. Some old promotions from mail.ru and ok.ru were blocked, too, but these have been dead as a doornail and irrelevant.
Note the smoking gun: regular Google links were used in configuration file on 11 March, and then were switched to firefox-tagged (sponsored) ones on 13 March (“part 2”). Why couldn't Mozilla switch Yandex search users to regular Yandex links the same way if they reportedly had “troubles” with their agreement, which supposedly would be the best for the users, and best for Mozilla, apparently? Imagine their wet faces when they beg on their knees for more after successful extension of Google's sponsorship agreement onto 4 big countries, and you'll know why.
The changelog for 98.0.1 is currently absent. The changelog for 98 has a vague note about search engines. The relevant code-related bugs have zero explanations. Bug 1748923 is private. The UI that alerts the user about the removed search engine was introduced two weeks ago (and had it text changed to be more vague). There is a specific (and vague) help article about the switch:
https://support.mozilla.org/en-US/kb/search-engine-removal
However, the first signs were noticed about a month ago (in Russian):
https://www.opennet.ru/opennews/art.shtml?num=56721
As usual with Mozilla, there were bugs. Users of previous version of Firefox that automatically update the list of search engines but have no UI to warn them about the changes got it removed silently.
I personally don't use Yandex, its results for IT-related English language queries have been quite shitty, in all honesty, for many years, most likely because of the supposed full-on focus on keeping the existing (mostly non-English-speaking) users. However, it's another nice example of treating people like swine by those who have even a tiny bit of power over them. Oh, we're used to that, thank you very much.