I mean simply like this:

:root{ 
  backdrop-filter: brightness(99%);
}

Alternatively html selector would also work to refer to the document root element - about:newtab is a html document after all.

The same will also happen with filter, transform and a few other properties. The reason is that when these properties are applied then a new containing block is formed, and that will cause fixed and absolute positioned children to behave somewhat unexpectedly in they now “appear to reserve space” whereas normally they don’t. In this case the newtab options menu is fixed positioned, and thus if the body has backdrop-filter then the menu box causes the page to overflow.

There is an exception to that mentioned containing-block formation though; if you apply the property (such as backdrop-filter here) to the document root element then no extra containing block is generated. I suppose it should be pretty simple to just apply your rule to :root instead of body.

Right, that makes perfect sense. The property being a variable nor it having an important tag are not meaningful to explain what’s happening here.

What is important is simply what the address of the .css file is which sets the background-image property, because relative url resolves relative to that. The internal style sheet where this background-image property is set is chrome://activity-stream/content/css/activity-stream.css. So if the url it uses is ../newtab/wallpaper-dark.png (as by you setting the variable as such) then it will try to load an image from address chrome://activity-stream/content/newtab/wallpaper-dark.png which surely doesn’t exist.

But if you set background-image property from within userContent.css then the relative url resolves relative to that instead.

Yeah, loading any external resources - I would think fonts even more so than images - is potentially risky. In addition, there is somewhat realistic possibility for browser fingerprinting if you use some style that makes browser viewport size atypical.

I can’t think of any actual security issues other than those. Otherwise the worst that a style could do is crash the browser, to make it utterly unusable or make it super slow. But those are all recoverable by simply trashing userChrome.css and restarting Firefox.

Sounds like what’s happening here is that the relative path you have given doesn’t resolve to what you think it does. See, the variable --newtab-wallpaper-dark is used by an internal style sheet, so when the url() actually gets resolved it will be relative to the internal style sheet address - not to the file path of userContent.css.

So, a workaround would be to give it an absolute file path or override the property where that variable is actually used.

Yeah, history is extremely valuable feature. I think I would rather get rid of bookmarks and maybe even tabs rather than history.

Fancy. Quite an artistic layout for sure.

In that case the issue is likely that files on disk are being modified by whatever mechanism your IT uses to push updates to devices. If the program files are modified while Firefox is running then you will unavoidably get this prompt.

I suppose the best you can do is to ask your IT folks to not update programs that are currently running.

I have no clue about hacking macOS

Take this with a grain of salt, but I believe I’ve read that standard backdrop-filter won’t work here and this would instead require OS compositor level mechanism because menupopup and panels are technically separate windows (or window-like widgets) from OS perspective.

Yes. I want to have access to both history and bookmarks on all my devices and send specific tabs to other devices. Sync makes these super convenient.

It is. But note that that site shows code in mozilla-central repository, i.e. code for nightly which can change quite often.

You can switch to release repository by replacing the mozilla-central part of the url with mozilla-release

The path they can be found from depends on which component they belong to, but bunch of them can be found over here at searchfox (this folder should mostly correspond to what icons are found inside chrome://browser/skin/).

No lie, the reddit nick was annoying at times

Hi, I can’t help you too much, but I can tell you what little I know.

I honestly have no idea what has been happening in reddit since last July. As far as I’m concerned the whole platform is dead, killed by admins, and I don’t want to have anything to do with it.

Indeed, the sub was abandoned some years back as well, back then I became a mod solely to rescue it from abandonment. I believe (although I might remember incorrectly) yawn_zz was given mod rights first by admins, and then I was given mod status by them. No clue how this sort of deal would resolve nowadays.

At any rate, I don’t have any other contact with them, but without going to any further details I can say that our relationship was suboptimal.

Frankly, the community was less willing to move to another platform than I had hoped, but perhaps now is a good chance to try again :)

I can’t remember of the top of my head, but I seem to recall the custom highlighter is inserted to the end of the dom in targeted window, so maybe you can find it just from the inspector.

But I don’t think it will do you much good because it sounds like you would need to modify native c++ code to make the changes that would allow the current highlighter implementation to work with panel or popups. At least, that’s how I understand it.

I think this is essentially bug 1728912 - maybe you want to read and/or follow that.

Huh, okay. That does seem like extensions can indeed write arbitrary data to default download directory automatically. Maybe you can create a feature request to existing extension for that kind of feature. Although I would think that an interval is totally unnecessary and it would make more sense to only make a backup whenever style editor is started or a style is otherwise being added/removed or some such.

Bleh, I’ve used way too generic selectors there… I updated the style and now it only applies to #main-menubar not all menubars like before. Another alternative could have been to use @-moz-document block to limit the style to only browser.xhtml but it’s probably alright like this.

I’m not 100% sure about this, but I don’t think addons are allowed to write anything to disk without user interaction, except to their own database of course.

I guess you could do it if you had some other installed software that backed up extension data from Firefox profile folder. Then again, if you are worried about some profile data getting wiped, then it would just make sense to back up the whole profile and not just little bits of it because who knows what other data could get wiped.