Just wanted to leave a note here as well that QA has finished the remaining round of testing on Firefox 125 MSIX build and we did not found any new issue. For the situation that 1-click default works will set everything that's intended as default and for the cases where 1-click default does not work, after manually setting the browser as default it does what's intended.
Bug 1882413 Comment 6 Edit History
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Just wanted to leave a note here as well that QA has finished the remaining round of testing on Firefox 125 RC MSIX build (125 beta was already tested previously) and we did not found any new issue. For the situation that 1-click default works will set everything that's intended as default and for the cases where 1-click default does not work, after manually setting the browser as default it does what's intended.