Which Linux?

[Agent_24]

Which Linux version you guys recommend?

The answer to that is a very big "It depends" - but based on your other questions, I personally would suggest Linux Mint.

The great thing about most Linux distros these days is they can all be run as a "live system" and booted from CD/DVD or USB. It runs entirely in RAM and doesn't touch your hard drive. You can use this to try out the system before you even install it, and decide if you like the feel for it or not. You may wish to try out several (especially some various desktop environments) to see what you think.

May switch from Windows to Linux. Which version is closest to Windows for user who never used Linux.

They are completely different systems, no version of Linux is close to Windows internally, but what I assume you mean is "which version looks and feels closest to Windows"

This again, depends on what your experience with Windows is, and which version of Windows you like best... Linux Mint (at least the versions I've tried) feel similar to Windows 7. But the graphical interface (desktop environment) and themes can be changed if you prefer something different.

Bear in mind it will never feel or look 100% exactly like Windows. Think standard petrol car vs battery EV. Similar, but will never be the same.

Main uses:  Web browsing, emails, simple Word processing.  Have been using Firefox for a years and would like to continue using it. 

Firefox can be used on Linux with no problem. So can many other browsers. For email there is Thunderbird and other clients exist. Webmail will of course work in your browser. If you want simple (and even complex) word processing, LibreOffice (which comes with a lot of Linux distributions) works great.

Can it run WhatsApp?  I need to run this from this machine.

No idea, but it seems someone else has possibly answered that question in a previous reply.

Is Linux as user friendly as Windows?  Worse or better or pretty much the same?

Again, that depends. If all you do is install Firefox, Thunderbird, and LibreOffice, and just use those as you would normally, you shouldn't run into any real issues on a stable distro. Some more bleeding-edge ones have bad updates from time to time and cause problems, though this is no different from Windows, when Microsoft releases garbage updates here and there.

If you stick with a stable distro it's very unlikely you would ever get such issues.

Easy to install?  My current PC is about 10 years old, can it handle Linux?

Yes, most distros come with a very easy to use graphical installer that guides you through the installation just like modern Windows installers do. Even a 10 year old PC should be fine, unless it was some super low-end one, in which case you might get performance issues, but Windows would also run poorly on such a PC anyway.

Is it FREE or needs to pay?

Most general Linux distros that are aimed towards home users are free. The ones that are paid are usually enterprise grade, which you wouldn't be using anyway.