As announced in early 2023 when selecting website links from within the Microsoft Office Outlook client they will open using the newer Microsoft Edge browser (i.e. the modern replacement for Microsoft Internet Explorer).
As outlined by Microsoft, Inc. the change in default browser for web links viewing is designed to improve viewing of websites to have a “single viewing experience”.
How to revert website links to open in default browser:
...
Microsoft has made Microsoft Edge the default browser for opening web links in Microsoft Outlook. Web links now open right alongside the email they’re from in the Microsoft Edge sidebar pane. This change is designed to provide users with a more seamless experience between email and web content in a single, side-by-side view.
If you prefer that web links open in your chosen default web browser, instead of Microsoft Edge, you can adjust your Outlook settings by following the steps below:
From you Microsoft Outlook home screen, select the File menu in upper-left hand corner).
Then, choose Options from the left-hand panel, choose Options.
Within 
Select Advanced from the Outlook Options screen on left-hand side, select Advancedmenu.
On
the Advanced options screen there will be a section for “In Advanced options, scroll down to the File and browser preferences”.
Within this area will be a choice for “ section. You will see an option labeled Open hyperlinks from Outlook in:” and options of either “. From the dropdown menu, you can select either Microsoft Edge” or “Default Browser”
Choose selection accordingly and then select the “OK” button to save changes. -
If you select
Microsoft Edge, then web links will continue to open in Microsoft Edge.
If you select
Default Browser, then web links will open
in your default web browser of choice.
(i.e. Mozilla Firefox, Google Chrome, etc.)
Once you make your selection, be sure to select the “OK” button to save changes.