I think someone wanted to make fun of you, so it was a joke:) But don't worry! I found the describtion how to do this(if you will need this in future who knows) or you can do this vice versa to return :

You can try to use Style sheet Style sheet can be downloaded at this address=> https://github.com/wesbos/aprilFools.css/blob/master/aprilFools.css. It was created for use in the Google Chrome browser, but it can also be used in Firefox and Opera, although you will need to make some changes, the description of which you will find a little below.

The file contains not only the possibility of turning the pages upside down, but also other funny tricks. You only need to find in the text the name of the desired effect and allow its application. This is done as follows:

1. Open the downloaded file aprilFools.css in any text editor.

2. Find the name of the desired effect in the text, for example upside down (upside down).

3. Uncomment it, that is, remove the / / signs.

So you had / * Turn every website upside down

• /

body { / * - webkit-transform: rotate (180deg); * / }

Should be: / * Turn every website upside down

• /

body { -webkit-transform: rotate (180deg); }

This browser does not support special styles out of the box, but you can use the Stylish(https://addons.mozilla.org/en-US/firefox/addon/stylish/) extension. Just install this extension and create a new style in which you copy the contents of the downloaded file. Just before that, remove all references to “-webkit-” in general. Thus, from -webkit-transform: rotate (180deg); you will just transform: rotate (180deg); .

Wait... I haven't visited any special website, and I haven't downloaded a single file. What I did was using Firefox' own update service yesterday, before I shut down the computer. And when I started it today Firefox displays everything upside down. Where would the css file come from in that case?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

What's your computer system and Firefox version?

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

I have Windows 10 Pro, Firefox Quantum 68.0. Computer is an AMD Ryzen 5 1600, ASUS motherboard, GeForce GTX 1050 Ti. Graphics drivers and Windows are up to date.

Tried to reinstall Firefox but the problem persist. Firefox safe mode rotates the browser as it should be, so I removed the two add-ons I used (NoScript and Social Fixer). But the upside down screen in normal mode still persist.

It could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Disabling Hardware Acceleration fixed the problem. Thanks!

Glad to help. Safe Surfing. Very, very odd, however.