On the regular .exe version, Firefox has an easy to access "Check for updates" button right at Help-->About Firefox Window. MSIX version understandably doesn't have this since Windows Store should care about the Firefox.
What about adding the "Check for updates" button back which just links to Windows Store Firefox URL? ms-windows-store://pdp?hl=en-gb&gl=tr&ocid=pdpshare&referrer=storeforweb&productid=9nzvdkpmr9rd&storecid=storeweb-pdp-open-cta&webid=f8bfda58-0529-425b-b22b-1266a22d7f06&websessionid=29043861-30a2-4528-a958-2d3c4d241bac
For example, there could be a emergency update or some specific update regarding the users configuration and it can be easily updated that way.