I upgraded to Debian unstable because firefox was broken recently, but that broke OpenCPN, which is quite important for sailing. :)
OpenCPN package wanted libcurl3, but I now had libcurl4, and apt wouldn’t install both of them.
I solved it by building OpenCPN from source. The latest version seems to work with libcurl4. These (https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn:developer_manual:developer_guide:compiling_linux) instructions worked, but then I had to get the harmonic files for tides. Got them from the github repo and put them in /usr/local/share/opencpn/tcdata/, not /usr/share as it said somewhere. You can configure it in settings, charts, tides & currents anyway.
There is an assertion failure on startup but it seems to work.