The search bar might be what you want. There is an option to turn it on in the settings.
Firefox
/c/firefox
A place to discuss the news and latest developments on the open-source browser Firefox.
Rules
1. Adhere to the instance rules
2. Be kind to one another
3. Communicate in a civil manner
Reporting
If you would like to bring an issue to the moderators attention, please use the "Create Report" feature on the offending comment or post and it will be reviewed as time allows.
I was able to enable it on stable by toggling browser.urlbar.showSearchTerms.featureGate
Edit: Nevermind, it doesn't work for custom engines, which suggests that it's a separate implementation from mobile?
it doesn’t work for custom engines
It's working with Kagi for me; added it as a custom engine as their docs recommend.
I'm not sure I fully understand your requirement so this may not be what you're looking for but CTRL+k brings you straight to search and puts the cursor in the URL bar. The engine is to the left and can be selected.