https://profiler.firefox.com/ front-page should disable its animations (on front page and loading screens) for users with `prefer-reduced-motion`

[dholbert]

https://profiler.firefox.com/ plays a "slide-in" transition on page-load (under some CSS classes named homeTransition and similar).

We should put that behind a prefers-reduced-motion media-query check:
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion
...to make this page more accessible for users who've got that flag set (for whom this animation may cause nausea/migraines).

(note: this issue was originally reported at https://bugzilla.mozilla.org/show_bug.cgi?id=1761087 ; I've moved it over here on behalf of the user, since I believe this is where profiler.firefox.com bugs should go.)

[dholbert]

While we're at it, we should probably use the same media query to guard the "colorful-sliding-bars" animation on the "Downloading and processing the profile..." loading-screen, too (the screen you see when you visit the URL of some particularly-large profile).

As well as on the "Importing the profile directly from Firefox..." screen (the screen you see right after capturing a profile), which has the same colorful-sliding-bars animation.

[dholbert]

(CC @canova who might be interested to take this or might know who could.)

You can activate a prefers-reduced-motion configuration locally by creating a new about:config pref called ui.prefersReducedMotion, with the integer value 1; that turns on the feature and it should make the media query start matching for you.

[canova]

Hi @dholbert, thanks for filing it here!

Indeed we should remove those animations for prefers-reduced-motion. I've actually removed some animations a few months ago for it (#3895), and we removed more in the past, but apparently these are overlooked. I will remove them as well.