On Ubuntu with stock Firefox 67.0.1, KeePassXC-Browser 1.4.6, and KeePassXC 2.4.1 I'm getting the message "You use an old version of KeePassXC. Please download the latest version from keepassxc.org.". It does appear 2.4.2 has been released, but wasn't able to install it using neither the PPA repo nor the snap package. Would an upgrade even resolve this?
This warning is shown because the extension always checks for the latest version released from GitHub. An upgrade will not show the message. You can also disable it by setting Check for updates to "Never" from the extension settings. Even if the warning is shown, everything works normally.