Mozilla's latest Firefox release is deemed a major one with upgrades to the browser's policy engine for deployments in enterprise environments, Quantum CSS to render browser UI, support for Web Authentication API, and more.
GIMP 2.10 originally focused on porting the program to a new image processing engine, GEGL, but the new version also offers improved tools, better file formats support and a plethora of improvements targeted at digital painters and photographers.
A very popular open source tool for network analysis, this latest version opens the spectrum even more with support for 45+ new protocols. Check out the new Wireshark 2.6.0 here.
VLC is one of the most popular media players in the world and this new version has quite a few improvements for all platforms. Chief among them is enhanced hardware decoding for macOS which is now faster and supports more samples while reducing crashes.