We already know Google blocks Antivirus and third-party software from injecting code into Chrome browser, Mozilla is working on that now in Nightly with a new launcher Process that blocks a known list of DLLs that cause the problem to Firefox. In Firefox 66 Nightly, the launcher process is enabled by default and about: support shows the information about its status and the preference
browser.launcherProcess.enabled for that in about:config.
Mozilla has started work on mitigations for DLL injection two years back, The launcher process which has been created to tackle dll injections has been integrated into the browser process. In a wiki page, it was noted that “To block DLL injections into Firefox, we’re making some changes to how Firefox starts up. The launcher process is part of that and was just enabled by default on Nightly”.
The company is planning for the “Windows launcher process to ride the trains”. Accessibility software also is known to cause issues to Firefox browser, that’s why the company has added an option to settings to prevent accessibility services from accessing Firefox browser, which you’ll find under Permissions of Privacy & Security Settings.
Applications injecting code into the browser is common, but it always has caused stability problems to browsers, which Google has handled and fixed on Windows for their Chrome browser to a certain extent, now Mozilla is working to prevent third-party software especially antivirus from causing issues to their Firefox by injecting DLLs.
UPDATE: Aaron Klotz from Mozilla working on launcher process said in a reddit thread that they’re testing the process and planning to run it as an experiment in version 67. And initial rollout of experiment will not block any new DLLs apart from the ones they’re already blocking in Firefox.