1. This tutorial is very simple and tested on latest Firefox build 126.96.36.199. Run your Firefox and type about:config in the address bar and hit Enter button and accept the warning "I'll be careful, I promise!".
2. In the filter field type gecko and double click the entry gecko.handlerService.allowRegisterFromDifferentHost to change the value to true.
4. You will see an Add Application bar. Click on it to save the option.
Now you need to make Hotmail as your default mailto service. Go to Firefox – Tools – Applications. Scroll down to find mailto. In the drop-down menu to the right, select "Use Hotmail".That's all. [similar post]