I had to make the opposite change. Right-click in the search bar and untick/uncheck "Check Spelling". That will hopefully stick.

I had to make the opposite change. Right-click in the search bar and untick/uncheck "Check Spelling". That will hopefully stick.

I tried that, it seems like it's a second type of spellcheck, because it's like when that spellchecking is on the two different underlines are layered, possibly some sort of other more outdated spellchecking?

Is it possible MacOS is somehow involving itself?

Is it possible MacOS is somehow involving itself?

I origionally thought that, but it seems to only be happening for google, i've tried DuckDuckgo and bing, both of which do not have this problem.

Okay, turns out it's a feature of Google's search pages. You can suppress it by applying a custom style rule to Google pages using either the Stylish extension or a userContent.css file. Stylish is easier for setup and tweaking, but if you already have a userContent.css file, see #5 below for the code.

(1) Install the extension from https://addons.mozilla.org/firefox/addon/styl-us/

(2) Click its "S" toolbar button and then the Manage button at the bottom

(3) In the left column, click the Write New Style button

(4) Enter any name you like in the name box in the left column

(5) Click in the box on the right and paste this code -- Stylus should pop up an Import form:

@-moz-document url(https://www.google.com/), 
               url-prefix(https://www.google.com/webhp), 
               url-prefix(https://www.google.com/search){
    /* Suppress spell-check marking in the search form (22 Sept 2021) */
    form[action*="/search"] span {
        background-image: none !important;
    }
}

Click "Overwrite Style" and then click the Save button in the left column. (First attached screenshot.)

(6) Test on Google... (Second attached screenshot.)

Success?

Note: This kind of site modification is subject to future failure when Google changes how it does things...