Identical to v0.5.0 in code and behaviour. Republished under a new version number because Firefox AMO disallows reusing a version once it has been uploaded and deleted (v0.5.0 was pulled during testing).
If you're upgrading from v0.5.0 there is nothing new to read — see v0.5.0 release notes for the changelog.
Install
Chrome / Edge / Brave: download immich-companion-chrome-0.5.1.zip, unzip, then chrome://extensions → enable Developer mode → Load unpacked → pick the unzipped folder.
Firefox: download immich-companion-firefox-0.5.1.zip, then about:debugging → This Firefox → Load Temporary Add-on → pick the zip.