In Windows 10, it may happen at some point, that you can’t set the default browser correctly (Chrome, Firefox, etc). Here is one possible solution to get your system to remember your default browser choice, if you are having this problem.
Firefox
Paste Special in Firefox: Fix Conflict with AdBlock Plus Add-on
In your Firefox browser, if you cannot use CTRL+SHIFT+V to paste plain text without any formatting (paste special function), then one possibility is that you have a conflict with a Firefox add-on. A common conflict may be with the popular Firefox add-on AdBlock Plus. Edit preferences in Firefox to quickly fix this Firefox paste special issue.