Took a number of tries to figure out why people are having issues.
We all have personal preferences and it appears my preferences made this easy. For me. For some of you ... not so much.
The "site:" search mechanism turns out to be a Google syntax mechanism, not Firefox.
I always use Firefox and Google in combination since the two tend to be much more powerful in combination than other browsers and search sites. My preference.
The following works in MS Edge, Chrome and Firefox.
Using any browser, first search for the site "google.com"
On the mostly white Google.com page paste the syntax noted in the search box of Google (the one in the middle of the page).
Do not remove any of the spaces.