It can be retrieved by installing the Classic Add-ons Archive XUL extension (mentioned previously in this thread, as I recall ):
https://github.com/JustOff/ca-archive/#compatibility-and-installation
After successful installation, open "caa:addon/empty-cache-button/versions" in a tab; you would need latest "Version 2.7.1-signed.1-signed"; download the .xpi file to disk (storage of the file kindly provided by the Waterfox project) and manually install to New Moon 28 / Serpent 52.9.0 (both on the UXP platform) by dragging the file onto "about:addons"; you will be asked to restart browser to complete the installation; just a usage tip: be sure to close the caa:addon/* tab prior to restarting, else it would take quite a lot of time for the browser to reload post restart...
Yes, Version 3.4 (WE format) does install and function correctly in latest Serpent 55.0.0/moebius, but does not install/work in Serpent 52.9.0/UXP, because it requires WebExtension APIs not present in[/backported to] the UXP platform...
PS: if you like the "metro" style of the icons used in this WE version 3.4 of "empty-cache-button" (which, as said, can't be installed in either NM/Bk52), I have taken the liberty of "patching" the XUL version 2.7 (linked-to above, GLGPLv3 lic ) so as to include the "metro" icons (instead of the default "3D" ones); ergo empty_cache_button-2.8-pm27-28.xpi