Rated 5 out of 5 stars

Finally switching to Thunderbird for Mail, after using Mozilla Suite/SeaMonkey for ~20 years now, I'm very happy to have this extension available. I'm the contributor of the _"Partially collapse quotes"_ [1] feature to the old QuoteCollapse, so I want to point out this customization [2] alternative to the _"By default collapse only quotations that are in sub-quotations"_ option:

/* Establish predictable line-height */
:root, html, html > body {
line-height: 1.4;
}

/* Always show few lines of text of first level collapsed quotes */
blockquote[type="cite"]:not([qctoggled="true"]) {
max-height: 9.1em !important; /* 6.5lh */
}

/* Nested collapsed quotes show just one line of text */
blockquote[type="cite"]:not([qctoggled="true"])
blockquote[type="cite"]:not([qctoggled="true"]) {
max-height: 1.4em !important; /* 1lh */
}

[1] https://github.com/mjg/QuoteCollapse/pull/1
[2] http://kb.mozillazine.org/UserContent.css

This review is for a previous version of the add-on (4.0.2). 

Finally added this feature for addon version 4.1.0. See here:

https://www.thunderbird-mail.de/forum/thread/89536-quotecolors-collapse-version-4-0-f%C3%BCr-thunderbird-102/#post506394

Checkin on GitLab:
https://gitlab.com/ThunderbirdMailDE/quote-colors/-/commit/1032ee486586bbd01ebff913ae417837911d95e5