bhearsum2
This data comes from attribution data (https://firefox-source-docs.mozilla.org/browser/components/a...) and the installation ping (https://firefox-source-docs.mozilla.org/toolkit/components/t...).
Most of the Firefox installers/packages for Windows & macOS on https://archive.mozilla.org/ have a sentinel value for attribution data that indicates "this is a build that Mozilla produced and uploaded to archive.mozilla.org", and is considered to be from a known source. (Anyone that downloads directly from archive and redistributes will maintain this data and also be considered known.)
Downloads that flow through www.mozilla.org and meet certain conditions (most notably, Do Not Track being disabled) will have this data overwritten at download time with UTM parameter information (see the first link above). These are considered to be from a known source as well (our own website!).
I'm not an _expert_ on the analysis side of this data, but I believe that install pings that don't contain attribution data are considered "unknown". (There may be other cases that end up in the "unknown" bucket as well - I honestly don't know.) On Windows, there's a _shockingly_ high percentage of installs that fall into this bucket.
1) It will break partial updates, which means the first update attempt will fail, and then Firefox will fall back to a full update (larger, slower).
2) After the update is applied, yes - it will remove the tweak (because the enterity of omni.ja will have been replaced).
(I work on the Firefox install/update system, and previously CI & release systems.)
Things are much better these days, but there is still much inherent complexity in a project like Firefox.