¡Hola Oleg!

Staying in an old known vulnerable version of Firefox is a terrible idea as it exposes you to all these https://www.mozilla.org/en-US/security/known-vulnerabilities/firefox/

Please go to https://www.mozilla.org/plugincheck and verify your plug-ins are up to date.

Could you please tell us exactly which "plugins don't work correct with newer versions"?

¡Gracias!

Plugins are from other programs on your computer. You would have to go into their settings and disable auto updates.

Not Recomended ! !

By Plugins, you really meant Extensions I take it as Plugins is not normally Firefox version specific and Plugins refers to say Flash, Java, Shockwave etc.

Have you checked to see if the author has provide updates for the Extensions? It is not Firefox that is made compatible with Extensions but the other way.

Also if you do not want to use Firefox 42.0 Release then at least consider using the Firefox 38 ESR version. It only gets allowed stability and security fixes and not new features. The next version ESR will be based on will be Firefox 45.0 so the 38 ESR will be supported up till a update or two after Fx 45.0 Release.

The current version is Firefox 38.4.0 ESR as it came out when Firefox 42.0 was released. https://www.mozilla.org/firefox/organizations/all/

Compared to using the old vulnerable Firefox 34.0 Release the 38.4.0 ESR will be much more secure.

Yes, I meant extensions, you are right. But it doesn't matter. It's my choice what extensions and what version of firefox to use. I'm very tired of bad suprises afer updates and I'd like to use the version that works good enough. I have already gone into firefox settings and disabled auto updates, but the program updated nevetheless! Why this option doesn't work properly?!

The Mozilla Maintenance Service might be updating Firefox for you. Disabling updates in Firefox doesn't stop that "service" from doing updates. https://support.mozilla.org/en-US/kb/what-mozilla-maintenance-service

Thank you, I hope it will help!

My FIrefox 42 just automatically updated to 43. I use unsigned add-ons so I don't want my Firefox to update past 42. How do I prevent it from updating? What's Firefox's update URL (so I can block it in the host file)? Thanks in advance.

Press the Alt or F10 key to bring up the tool bar. Then Help > About Firefox will tell you what version have and check for updates.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.