First, do this:

How do you completely turn off update checks in Firefox v64?
You can use a policies.json file to disable updates for all users.
https://github.com/mozilla/policy-templates/blob/master/README.md
or
https://winaero.com/blog/disable-updates-firefox-63-above/

You can use a policies.json file in the distribution folder in the Firefox installation folder instead as also listed in the article, so no need to use GPO keys in the Windows Registry. • https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

Install Older Version Of Firefox {web link}
Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade.
Click the Directory of other versions and languages link.
Look for the directory of the version that you want.
But, remember that old versions may have security issues.

You should also check out
Firefox; Extended Support Release {web link}
ESR Notes
System Requirements

You should use the Selenium WebDriver (GeckoDriver) in current Firefox release.

• https://www.google.com/search?sa=N&num=100&q=selenium%20firefox
• https://en.wikipedia.org/wiki/Selenium_(software)#Selenium_WebDriver

• Selenium IDE https://addons.mozilla.org/firefox/addon/selenium-ide/
• https://github.com/mozilla/geckodriver
• https://github.com/mozilla/geckodriver/releases

Could you help me? (All machine translation) I now need to use the old version of firefox (Firefox version-65.0.2 64bit for linux ubuntu) because selenium only supports this version. But I can’t find a way to download this version How can I get this version? Thanks :-)

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements