Microsoft add-ins are web apps, ie they run inside a browser. The combination of your operating system and Outlook version is used by Microsoft to determine which browser the add-in is managed in (Edge or Internet Explorer (IE)).
Note, this is regardless of the browser you use on a day-to-day basis and ExtendSync does not control which browser Microsoft uses.
Some users have their browser set to Clear Cache whenever the browser is closed. If this setting is applied to the browser used by ExtendSync (IE or Edge) then this will force you to login on a more frequent basis. To avoid this set your browser to not clear cache on exit.
Microsoft Edge Instructions
Step 1: Settings > Privacy and Security.
Step 2: Choose what to CLEAR.
Step 3: Make sure that Cached Images and Files are set to OFF.
Microsoft Internet Explorer Instructions
Step 1: Settings> Internet Options.
Step 2: Make sure that delete browsing history on exit is NOT checked
