This is exactly what I needed to quickly search for vinyl records accross multiple stores and sites. Thanks!

The extension is too confusing, and the directions are not clear. I am not very tech savvy, so this extension definitely isn't for me.

N'a pas fonctionné : Firefox 84.0.1 Windows

Awesome. I initially didn't understood well how it worked, but the developer was really helpful, so I'll clarify my misunderstandings here for others in similar situation so that your simple searches work:
1.- You don't have to mingle with search preferences in Firefox.
2. Consequently, you won't prefix regular searches with non default search engines with "@" when doing - as you do in other explorers and FF defaults.
3. You do have to prefix these custom searches with "ms" before the keys added while doing the search. "ms" stands for Multi Search, but you have to use it even for a simple (non multiple) search. Think of ms as "My Search" if that helps fixing what you're really doing in your head.
4. What a real Multi Search (with several keys) does is opening multiple tabs, one for each search engine that you included as the key.

I mostly wanted to alternate searches, rather than doing multisearches at the same time (but will explore that in the future). If you want to search for only the last 3 years for example, which you can do in Chromium browsers with
https://www.google.com/webhp?tbs=qdr:y3

, you can do it in FF with this extension, as clarified by Ravi, the developer with:

https://www.google.com/search?q={searchTerms}&tbs=qdr:y3

You can substitute y with m, d... for months, days... and of course use the number you want.

Still don't know how to do this with ecosia or duck duck go. If anyone knows, please share it so that we all know too :)

Thanks a lot to Kevin for this so needed extension!

you dont need an extension for that. just right click any search field

I miss this SO much on Pale Moon. Any chance you make this possible?

Absolutely not clear as to how to use this extension. No icon, no options sections in the settings, no nothing.

not sure how to add it to the search bar where all the search icons are at (about:preferences#search)

+ Settings are clear and easy to modify.
- something needs to be done for the amout of tabs that a single command line opens. lets say I translate 5 words in a row a 5 engines, thats 25 tabs opened just like that. There is an addon Translation Comparison that has the multi-column view, so, in total youd have one window/popup but several search engines. or any other simpler trick might do as well.

Works great, but keeps focus after pressing ⏎. What can I do about it? It's rather annoying in web apps, which use keyboard shortcuts, because I have to type ⇥ and then the shortcut.

Excelente extensão