Step one: 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

Step two: Note: Some versions of Quantum change how the data files are set up, so they may not be backward compatible.

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

Warning: Installing a previous version doesn't fix most problems

Problems with an update are generally not caused by the new version of Firefox, but rather the update process. Installing a previous version will not help in most cases. The best option is to start a new thread about your issues, where others can suggest specific solutions.

Using old versions of Firefox also poses a significant security risk.

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

Old versions of Firefox are always available, but not recommended.

Where do you find the font to be too small and not bold enough?

On Windows 10, Firefox 89 generally uses the system font, "Segoe UI" and on mine at least, it is at 12px, which is 75% of the size of text in typical web pages. So the UI font is small from that perspective.

You have a few different options:

(A) Globally enlarge the browser text and images

This is like zoom for the user interface, and also zooms the content area. Would that be worth trying? Here's how:

The following setting governs both the content area and the "chrome" area (menus, toolbars, and tabs). If the content is too large after finding a comfortable size for the toolbar area, you can use a reduced default zoom level on the Options/Settings page.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste devp and pause while the list is filtered

(3) The layout.css.devPixelsPerPx preference has a default value of -1.0 which indicates following the Windows system-level scaling (on a large monitor, typically 100%). Double-click the preference and change its value to 1.5 -- this is the equivalent of 150% scaling. You can adjust as needed, for example, 1.25 for 125% or 1.33 for 133%. If something goes terribly wrong, there is a reset button at the right end of the row.

Note to any Mac users with Retina displays: start with 3.0 instead of 1.5.

(B) Increase Contrast using a Theme

Many users have found the lighter theme more difficult to read. You could try this one, which has colors similar to the old "Default" theme:

https://addons.mozilla.org/firefox/addon/photon-colors/

(C) Directly set user interface font sizes using custom style rules

This is a community-supported (unofficial workaround). If the above options don't help, we can dive into the details of using a userChrome.css file to modify the appearance of your Firefox.