Why does Windows 11 keep changing my default apps?

Windows 11’s supported default-app model requires a user selection, so a browser or PDF association should not silently change without an application update, policy, profile problem, or reset event. Open Settings, Apps, Default apps and configure the preferred browser for HTTP, HTTPS, .htm, and .html, then assign the PDF application to .pdf. Restart and verify each association. If the change follows an update to Firefox or the PDF application, repair or reinstall that application and record its version. On a managed PC, ask the administrator whether a default-association policy or imported association file is reapplied at sign-in. On a PC, test a clean boot and then a new local profile. Do not edit the protected UserChoice registry keys; Windows can reject unsupported changes. If Edge returns with no policy and the new profile behaves identically, submit Feedback Hub diagnostics containing the Windows build, update history, affected extensions, and reset time.