John K Jordan
In Firefox on Win10 there are buttons in the tool bar to increase/decrease the magnification (zoom level) of the screen using the mouse or trackpad. It shows you the percentage and clicking on the percentage number returns the zoom level to 100%. If not visible on the tool bar, the Customize option can put the zoom and other controls where you want them.
You can also use the Zoom menu (which shows you the control keyboard shortcuts.)
>>>Now I wonder if there is a quick & simple way to increase/louder the sound level????
To get a useful answer to many computer questions it might help to state a few things:
1 - the operating system
2 - the browser or other software in use
3 - the specific hardware
For example on my Dell Precision 7510 the Fn2 and Fn3 keys adjust volume +/-. Another computer has the same functions but on different keys.
When playing a video there are sometimes different keys for volume, sometimes the up and down arrows in youtube and VLC, sometimes other keys such as with Win Media Player, and sometimes the keys vary depending on which mode you are in.
You can google the words "volume" and "keys" or "shortcut keys" along with other things, like the browser name, the video or audio player software, etc. (or you can google "VOLUME" and "KEYS", etc. if your caps lock is sticking.)