Firefox color waaaaaay off

What the general public will see depends entirely on what kind of monitor they have, and whether they are using a fully color managed web browser or not. It's the combination of a wide gamut monitor and a browser without full color management that's causing the over saturation. 

You have no control over what monitors or web browsers other people use, so there's nothing you can do to prevent this from happening. And, this has absolutely nothing to do with css or html coding.

For Mac users, this is generally not a problem, except those who have a wide gamut monitor, and use Firefox with Color management mode set to 2. (which I believe is the default) All other major web browsers for Mac are fully color managed.

On Windows, things are different, since neither Internet Explorer nor Edge are color managed.

Users with standard gamut monitors (which is what most people have) will see more or less correct colors. Those who use wide gamut monitors with these two browsers will not only see over saturated css colors, but also over saturated images.

But those who use Chrome, Opera, or Firefox with Color management mode set to 1 will not see over saturation.

Most likely, Firefox is set to color management mode 2, so try setting it to mode 1, like I described above.

Double-click the line that says gfx.color_management.mode, then change 2 to 1 in the dialog, and click OK.

Then restart Firefox.

With the exception of Internet Explorer and Edge, all major web browsers are fully color managed these days.

This means that they not only color manage images, but also assign the sRGB profile to CSS colors.

Firefox is a special case, since it has three different options for color management, which are managed by typing about:config in the address field, and scrolling down to gfx.color_management.mode.

• Mode 0 = no color management
• Mode 1 = full color management, including assigning sRGB to untagged images (images without an embedded profile), and assigning sRGB to CSS colors.
• Mode 2=  color management for tagged images only (CSS colors are not color managed)

If you have Firefox set to mode 2 (or 0), CSS colors may not display correctly, and if you have a wide gamut monitor, they will display over saturated.

Windows users who use Internet Explorer or Edge on a wide gamut monitor will see all colors (images and CSS colors) over saturated.

Phones and tablets are not color managed, and can not be expected to display correct colors.

I export web images in Photoshop and convert to sRGB color space.  This ensures more consistent color management on the web. 

CSS colors  don't vary much on my computer.  However, I do notice fluctuations on other devices. But that's to be expected because they are not all calibrated the same way.