Netflix probably doesn't recognize ESR as opposed to Firefox 133.3 release when it checks for security to access their site. If so then this is Netflix choosing to do this and using ESR will not allow you to access the Netflix site properly. Which would mean you need to get the release version to use for Netflix or another browser aka Safari on Apple to use Netflix.

Sorry, the Extended Support Release of Firefox 115 is the last Firefox that will run on your OS (see: Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release). So I think you are updated as far as you can for now.

As a test, could you try setting the following value to see whether it fools Netflix:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported.

(2) In the search box in the page, type or paste forceVersion and pause while the list is filtered

(3) Double-click the network.http.useragent.forceVersion preference to display an editing field, and change the value to 128 then press Enter or click the blue check mark button to save the change.

If you try Netflix again in a new tab, does it still give you the same message, or does something else happen now?

In case the problem is not your Firefox version but your Widevine version -- that's the plugin for digital rights protected media -- could you check your current version on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Plugins. On the right side, find "Widevine Content Decryption Module provided by Google Inc." and click to view more details. Mine is showing:

Version : 4.10.2830.0 Last Updated : October 15, 2024

Is yours similar?

Which would mean you need to get the release version

In case you missed it, the OP is using macOS 10.12.6 in which Firefox 115 ESR (currently at 115.18.9esr) is the most current version to support it. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

Firefox 116.0 and later including the Firefox 133.0.3 Release as you suggest the OP to use requires macOS 10.15 (ten.fifteen) at minimum to run.