I am another user who found Old Search in a separate searchbar a lot more practical. I used it up until FF63 with CustomCSSforFx. Combining jscher2000’s suggestions and the pre-built CSS options in CustomCSSforFx, I could get a reasonably good simulation of Old Search.
But it’s not perfect and I still don’t like it. At least the default search engine icon display should be re-implemented (although I won’t hold my breath expecting it).
I also take issue with the attitude of developers, even helpful ones in this thread who are giving useful advice. Even if there are tweaks to restore old functionality, users shouldn’t be forced to do so. For relatively tech-savvy people like me, the issue is just the time I have to waste to find & correctly implement those tweaks. But for a lot of other users, especially older people, even just to find new options/tweaks and/or re-learning everything every two years is just too difficult. Developers don’t seem to realise that not only aren’t most users geeks, even most users who customize software aren’t geeks.
With Firefox, for me, the perverse situation is that I can’t switch to other browsers, because the changes in FF are aping those other browsers. SO I have to use whatever customisation is still available to get back old features like coloured menu icons.