While there was talk of Firefox 151 shipping a native JPEG-XL image decoder, that feature was punted to Firefox 152 beta. But there are updates to its built-in VPN support, private browsing improvements, various settings changes, support for merging multiple PDFs directly within the Firefox PDF viewer, and a number of new developer APIs.
One change worth noting on the Linux (and macOS) side is that local profile back-ups are now available with support for restoring them across platforms. This is for more easily transferring / restoring Firefox profiles on macOS and Linux without manually copying the hidden directories.
One of the new developer APIs in Firefox 151 is support for the Document Picture-in-Picture API. The Document Picture-in-Picture API allows for having an always-on-top window that can be populated with arbitrary HTML contents. The intended use-case here is for custom controls or details on video conferencing calls or similar behavior to provide a more featureful experience over the existing video picture-in-picture API support.
More details on this arbitrary HTML Document Picture-in-Picture API support "documentPictureInPicture" can be found via the Mozilla documentation . Google Chrome has been shipping this support going back to Chrome 116.
The official Firefox 151.0 release binaries can be downloaded as always from ftp.mozilla.org .