MrOtherGuy

These new profiles that can be accessed through menu are not exactly sub-profiles, but I think more like sibling-profiles forming a profile group. Profiles in the same group know about each other, so in the UI you can switch between them and I think there are some settings that can be shared between the profiles in the same group.

I don't know of any plans to deprecate the old profile system, nor about:profiles for managing them.

I would seriously suggest to at least check the last modified time of the file instead of constantly re-computing all the style rules every second. So at the very least:

// store last known modified time
var previousModified = 0;

setInterval(() => {
// ...
chromepath.append("userChrome.css");

// check last modified time before proceeding
var modified = chromepath.lastModifiedTime;
if(modified <= previousModified){ return }
previousModified = modified;

var chromefile = io.newFileURI(chromepath);
// ... 

That Clear cookies and site data... button is only visible if there is data to clear. These screenshots indicate that searx.drgnz.club is not storing any data currently in Firefox, but startpage.com is storing some data in Librewolf - that is assuming that Librewolf also hides the item when there is no data.

Go to about:config and create a new number pref with name ui.prefersReducedMotion and set the value to 0. Afterwards that Firefox profile should use animations even if they are disabled on OS. Works on Windows at least.

I run nightly, not exclusively but pretty close, and I must say Firefox' nightly builds are pretty damn stable in my experience. For sure, there are situations where some feature y is clearly unfinished, but it's super rare to face a situation where I would even need to think about working around some issue - such game breaking issues just don't happen too much at all. Usually, if a build is found to be truly broken (like crashes very often etc.) then nightly updates get paused. I can remember maybe two times that I've had to revert to previous build in over ten years because the I had received an update before updates were halted.

Go to about:config and set the pref browser.ml.chat.hideLocalhost to false Afterwards you can select localhost as the provider in Settings > Firefox Labs

F12 opens web developer tools - the console there runs scripts in the website context - you cannot use that to access browser internals like PlacesUtils.

You need to run your script via browser console, I can't remember a hotkey for it, but you can find it from menu > more tools... > browser console

Also, I'm not sure but there's a chance that browser console is "read-only" in release firefox - meaning you might not be able to run anything from it. If that is the case, then open normal web developer tools (F12) and go to its settings, there's some checkbox there to enable "browser chrome debugging" or something like that. After checking that (and reopening browser console) you can run your function from browser console.

How exactly are you trying to run your javascript? Website javascript certainly won't be allowed to create bookmarks. If you run the function on browser side however, then it should work fine - but then I don't understand why it's wrapped into javascript url.

If it's a javascript: url because you tried to run this as bookmark itself (ie. clicking this special bookmark creates another bookmark folder and a bookmark inside it) then that's not going to work because that's pretty much just user provided code running in website context.

That is not browser toolbox - just normal devtools. Browser toolbox is separate tool which is used to inspect the browser window itself rather than web content. It's essentially a separate Firefox instance with it's own profile.

That is not possible. Browser toolbox runs in a completely separate Firefox instance in a separate profile so there's no way you could display it inside the "main" browser window.

I don't think there a way to open the library to history section via address. Library window history and bookmarks section are the same document, and the buttons that open open library window to history view do it by opening the window with extra window arguments - which you cannot do by simply changing the url.

A possible other option to show history would be to open Firefox view to history section. about:firefoxview#history

There's also another thing that I don't think is mentioned yet. The options available in Settings are supported features. If the feature is only available via about:config then there's a good chance that it is not supported or tested configuration. It might work or it might not, at least not in all scenarios