It doesn't launch the new binary at all. When the current process wants to create a new process, instead of doing fork+exec (which launches the new binary and wreaks havoc because versions now don't match), it simply tells the ForkServer (a different process running the old binary) to fork (split) itself.

Chromium also does this; they call their equivalent to ForkServer the zygote process and this article explains it really well.

Did you read the article?

I already don't restart Firefox very often, maybe once a week, if I'm feeling like it.

• No More Forced Restarts: This is a huge relief for many!. Firefox on Linux no longer requires a forced restart after your package manager applies an update. This means smoother, uninterrupted browsing, even during system updates.
• Reduced Memory Usage: ForkServer leads to a significant reduction in memory consumption for content processes. The base resident memory for a content process is now around 50% lower. This helps Firefox run more efficiently, especially if you have many tabs open.
• Faster Process Startup: You'll notice that content process startup times are reduced by approximately 35%. This translates to snappier new tab openings and overall improved responsiveness.

I for one am extremely happy about the no force restart. It's quite minor in the grand scheme of things, but it is so annoying.