I tried to sign a document via the function File->Digital Certificate->Digital Certificate
The “Sign Document” Dialogue does not show the certificates I have in my keychain.
Alternatively I tried to use the Firefox certificates; they show up, but I still cannot sign.
“Start Certificate Manager” opens Keychain, but from there nothing more seems possible.
What type of certificates do you have?
Which version in particular, of Libre Office?
There are several known bugs with certificate signing:
tdf#159612
tdf#157724
LibreOffice Version is 24.8.2.1 (AARCH64), macOS Sonomna 14.6.1
I have several X.509 certificates, from providers GlobalSign, D-TRUST, and also self signed certificates created in keychain.
The bug tdf#159612 sounds similar to the problem I have.
Is there a chance this will be fixed? Or a known workaround?
… and you even replied there in the end of September 
Of course. There is always a chance. Sometimes, such a chance realizes soon; sometimes, it takes years. Many issues wait for their chance for decades.
But you can try to increase the chances: you may try to find out more details, that could help to reproduce or understand the issue; you may try to fix it, or hire someone to fix it.
I was the initial bug reporter for that bug, and there is no known workaround. For the moment, I have to export my court documents to PDF, and then sign them with my corresponding key in Adobe Reader.
The whole key signing thing on macOS was ignored for quite a long time and only relatively recently saw some improvement through the contributions of a volunteer. Historically, most LO problems that are specific to macOS, are resolved in fits and starts over the space of a number of years. There simply aren’t enough developer contributors to cover all of the particular aspects of Apple’s ever changing APIs and corresponding deprecation. LO’s macOS code is still quite old in many places, and this tends to show up as odd, or sometimes difficult to pin down, bugs.
Alex