I'd like to use DBeaver with SQLite, but the network firewall disallows external connections. So DBeaver cannot download the driver automatically.
Where can I download the SQLite driver for it and how can I install it manually in DBeaver?
DBeaver supports any database that has a JDBC driver. So, the best thing to do is to go to the SQLite people themselves - they have a list of such drivers here. The page itself is quite old, so don't worry about the dates - just check around and see which one best suits your needs/is supported. Just as a matter of interest, SQuirreL SQL also works via JDBC - it's pure Java and is, IHMO, more mature than DBeaver, but YMMV.
Meanwhile I found where to get the drivers for offline installation: http://dbeaver.jkiss.org/files/ It seems this link is hidden.
The current version of DBeaver wants to contact xerial.org for downloading the driver. This site leads to the project's github page, where you can also download the driver: https://github.com/xerial/sqlite-jdbc