RT 4.4.3 Caching issues for squished js / difference between firefox and chromium

i have been looking into some performance issues with firefox (Debian Bullseye) to an Request-Tracker 4.4.3 on Ubuntu Focal (Ubuntu Package).
After some diagnoses i find the issue is that Firefox 78.10.0esr-1 does not cache this url although all headers look sane:



|Accept-Encoding|gzip, deflate, br|
|Authorization|Basic removed|
|User-Agent|Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0|


|Cache-control|max-age=2592000, public|
|Content-Type|application/x-javascript; charset=utf-8|
|Date|Mon, 03 May 2021 12:12:54 GMT|
|Expires|Wed, 02 Jun 2021 12:12:54 GMT|
|Keep-Alive|timeout=5, max=93|
|Server|Apache/2.4.41 (Ubuntu)|

I see a valid Expires, I see a valid Cache-Control. Still Firefox reload the same file over and over. Using chromium it caches the content. I am a little puzzled.

This adds up to 15 Seconds per click load time which is obviously VERY noticable.