Rust was a side project, asm.js was a side project that turned into WASM, IIRC LetsEncrypt was a side project (or at least a side project of Mozilla engineers, if not official), significant parts of the initial R&D for AV1 and Opus came out of Mozilla, Servo was largely a side project at the point that it got cancelled (the parts that had been largely successful had already been merged into Firefox, the rest was either nowhere near ready or being completely rewritten or both, and the main application for it at the time was some attempts to make a browser for running web apps in VR headsets at high framerates). Thunderbird was a side project.
But you don't see people complaining about those things, which largely didn't make Mozilla any money, you see them complaining about things that do make Mozilla money, like the branded VPN service. Or projects like FirefoxOS, which ultimately failed but still resulted in a lot of improvements to Firefox and web standards. And it wasn't a bad idea either, it just put the cart (mobile OS) before the horse (the web APIs that are needed to make a browser-based mobile OS successful, like access to GPS and IMU hardware, which didn't exist at the time) without the heft to really push through the barriers while competing directly with Apple and Google.
It seems like people define "frivolous side projects" as "anything Mozilla did that didn't end up being incredibly successful" and anything that did become incredibly successful gets taken for granted. "What did Rome ever do for us" rewritten as a tech parable.
(don't tell me that "we're going to create a whole new language to fix security issues in our web browser" didn't sound a bit frivolous in 2012)