Opening your favourite browser each morning does not necessarily make it the browser that Windows chooses for a link. A shortcut starts an app; a default association tells Windows which app should handle a particular kind of link or file.
Check the association that matters
In Windows 11, open Settings, Apps, then Default apps. You can inspect an application's associations or search for a link or file type. For an ordinary web address, check HTTP and HTTPS. A local HTML file and a PDF have separate associations worth inspecting only if those are the items opening incorrectly.
Mozilla's current guidance for Firefox also points to Windows settings when the browser's own default-setting action does not complete the change. Follow the controls present on your PC; wording and available shortcuts can vary by version.
Repeat the original action
Choose a harmless web link in the app where the problem occurred. Note which browser opens before the change, adjust the relevant association, and open the same link again from that app. Typing the address into a browser you have already opened does not test the handoff.
If only one app still behaves differently, check that app's own link-opening preferences or help. Record whether it opens a separate browser window or an embedded view. That distinction is more useful than repeatedly resetting every default application.
Keep browser data separate from the choice
Changing the default does not itself move your bookmarks or website sign-ins. Check the destination browser's profile before opening a work document. Leave PDF and other file defaults alone unless you also want to change how those files open; a browser preference is not a reason to replace every association.