1) You can open two windows side by side.

2) The default anything is set via user settings. If each user should be using their own user profile.

FredMcD said

You can open two windows side by side.

When you have one window open, hold down the Shift key to open a second window - press the Windows key on your keyboard + the right arrow key and the second window will open at the right side. Go back to your first window, hold down the Windows key + the left arrow key. You will now see both windows side by side - the content of each page that would normally be on the right side, you will now find when you scroll down.

Caleb said

if i make Firefox my main browser does that mean it will be the main browser for my mom too ( she owns the computer and it's a windows ten and right now she has google chrome)

If your mom is okay with using Firefox as her default browser, then you (or she) can set Firefox as default browser.

https://support.mozilla.org/en-US/kb/make-firefox-your-default-browser

But if you will both be sharing the computer, and it is hers; you can use Firefox and let your mom just use her default browser. On the same computer you can use different browsers.

You will both have your own account and your own profile, but that doesn't mean that you have to each use the same browser.

Re split screen :

If the earlier suggestion doesn't work for you (like you mentioned in a new thread you started); maybe you'd like to give this add-on a try :

https://addons.mozilla.org/en-US/firefox/addon/side-view/

But just opening two windows side by side (as mentioned before) works like a charm, once you get the hang of it.