An O.S. can be an operating system as well as this type of software like Firefox, GIMP & Thunderbird
[DOC] [TLE] Make Thunderbird the Default Mail Client | Thunderbird Help [PAR] If Thunderbird is specified as your system's default email application, it will ... 
when you click an email address link (like "email@example.com") on a web page 
or ... Thunderbird can be configured to check if it is the operating system's default 
... as Firefox on KDE Linux), the application doesn't check the operating system... [DOC] [TLE] Introduction to Linux, Free Software and Open Source | Linux Australia [PAR] This is a general guide to free software and open source (FOSS), as well as the 
Linux (or ... How can I find, install and remove FOSS applications?