User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:83.0) Gecko/20100101 Firefox/83.0

Steps to reproduce:

1. add a big element that largely overflow a fill page container in all direction with position absolute.
2. apply small blur on it.
3. add overflow hidden to parent container

<div class="out">
  <div class="in">
    A
  </div>
</div>

.out {
  position: fixed;
  top: 0px;
  left: 0px;
  right: 0px;
  bottom: 0px;
  overflow: hidden;
}

.in {
      background-image: url("http://via.placeholder.com/400x100/000070/FFFFFF/?text=Test");
  background-size: cover;
  position: absolute;
  background-position: center;
  top: -10px;
  left: -10px;
  right: -10px;
  bottom: -10px;
  filter: blur(5px);
}

firefox: https://i.imgur.com/V2LOETx.jpg
chrome: https://i.imgur.com/bQ2X6Hu.jpg

Actual results:

The cropped element have white blur at border even that should be out of viewport.

Seems the blur was apply after crop instead of before crop.

Expected results:

The cropped element don't have white blur at border because that was cropped (just like chrome)

Note: this is a regression, does not happen on firefox 80

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=25a50264ae3204f2df9add308fc712df45947637&tochange=c8102686489e51edec8a976f013398412577be44

Looks this may be a duplicate of bug Bug 1661147 .
Not sure whether they a regressed by the same commit.

This seems to be fixed for me in current nightly - could you verify?

(In reply to Glenn Watson [:gw] from comment #5)

This seems to be fixed for me in current nightly - could you verify?

I seems work incorrectly when opening the devtool for some reason.
Work if the devtool not opned.

This is working for me, both in normal mode and also with devtools open. It's likely it was fixed as part of the changes in bug #1749380.

Please re-open if you can still reproduce.