In Firefox you can combine JS bookmarklets, keywords and params to do something like this:

javascript:(function(){
    var args = '%s'.split(' ');
    alert(args);
})();

Useful example:

javascript:(function(){
    var args = '%s'.split(' ');
    var subreddit = args[0];
    var search = args[1];
    document.location.href = "https://www.reddit.com/r/" + subreddit + "/search/?q=" + search + "&include_over_18=on&restrict_sr=on&t=all&sort=new";
})();

Bookmarklet format:

javascript:(function() {var args = '%s'.split(' ');var subreddit = args[0];var search = args[1];document.location.href = "https://www.reddit.com/r/" + subreddit + "/search/?q=" + search + "&include_over_18=on&restrict_sr=on&t=all&sort=new";})();

If you assign the keyword redditsearch to that bookmarklet, you can type redditsearch PixelArt zelda on the firefox navbar and you will be reditected to the Reddit search for ‘zelda’ on r/PixelArt.

In general this makes the navbar a very powerful command line in which you can add any command with multiple params.

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It seems Mozilla has plans to get rid of this feature, see the ticket Migrate keyword bookmarks into about:preferences managed Search Engines. The good news is that the last comment, besides mine asking them not to remove this functionality, is from some years ago. I hope they change their mind, or forget about it…

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TIP: If you don’t want to remember the param order, you can also ask for them with a prompt if no arguments are specified:

javascript:(function(){
    var args = '%s'.split(' ');
    var subreddit = args[0] != "" ? args[0] : prompt("Enter the subbreddit:");
    if (!subreddit) return;
    var search = args.length > 1 ? args[1] : prompt("Enter the text to search:");
    if (!search) return;
    document.location.href = "https://www.reddit.com/r/" + subreddit + "/search/?q=" + search + "&include_over_18=on&restrict_sr=on&t=all&sort=new";
})();

Bookmarklet format:

javascript:(function(){ var args = '%s'.split(' '); var subreddit = args[0] != "" ? args[0] : prompt("Enter the subbreddit:"); if (!subreddit) return; var search = args.length > 1 ? args[1] : prompt("Enter the text to search:"); if (!search) return; document.location.href = "https://www.reddit.com/r/" + subreddit + "/search/?q=" + search + "&include_over_18=on&restrict_sr=on&t=all&sort=new"; })();

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Sorry for the reddit examples, this was posted originally on r/firefox some time ago and adapting them to lemmy seemed a bit too much work :P.