[Affected versions]:

• 65.0b9 build1 (20190107180200)
• 66.0a1 (2019-01-08)

[Affected platforms]:

• Windows 10 x64
• macOs 10.12.6
• Ubuntu 16.04 x86

1. Launch Firefox
2. Go to https://www.youtube.com/
3. Right click on any element and select the Inspect Element context menu option
4. For the "element" selector look for the "font-size" (or "font-family") declaration and change the property name in a invalid one

• inspect the Changes tab
• inspect the Rules section

[Expected result]:

• the declarations are disabled and marked as invalid

[Actual result]:

• the declarations are removed

[Regression range]:

• it seems that bug 1447736 introduced this issue
• pushlog: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=3d0acce0b1de3581e0c584dd37be696f4bf1bb52&tochange=378a8a64401f765bfd0706ff678a4f5db7c05385

Forgot to attach the screencast for the mentioned issue: https://drive.google.com/file/d/19QERyoYgoq7uh3rdz79cuBoHjHjMjren/view?usp=sharing.
Also, another note: after the affected rules are removed, this fact is NOT reflected in the Changes tab.

Thanks for filing!

Clarification for the person who's going to address the bug: once editing declarations inherited from an inline style attribute (this is what's meant by the "element" selector in the steps to reproduce above), they get removed from the Rules view if the property name is invalid. Invalid declarations inherited from other selectors are not impacted.

Happy to take a patch in nightly 67, or potentially, in beta 66 for this.
I'm marking it fix-optional to remove it from weekly regression triage, since it has a priority assigned.