Luckily, you might read this post fromMiniToolto find some useful methods to fix the annoying issue.
The SSL_ERROR_RX_RECORD_TOO_LONG error alert from Firefox is brought on by the misconfiguration on the server-side.
Now, I will provide more details about the reasons.

Also see:What Is TLS and How to Enable It on Windows Server?
Then, lets take a look at the methods to fix the SSL_ERROR_RX_RECORD_TOO_LONG issue notification.
Since this method has proved useful to many people, you’re able to have a try.

Thus, you better try disabling add-ons in Firefox.
Here is how to do that:
Step 1:Navigate to theFirefox URLbar.
Typeabout:addonsand press theEnterkey.

Step 2:Locate your add-ons and click theDisablebutton next to them.
Step 3:Finally, kickstart your web app and see if the issue has been fixed.
Also see:Firefox Not Responding?
The SSL_ERROR_RX_RECORD_TOO_LONG Firefox error can be because of incorrect proxy parameters.
Its recommended to check them and you might follow the steps below.
Step 1:you could press theWindows+Skeys at the same time to open theSearchbox.
Step 2:nudge the best matchInternet Optionsfrom the search result.
Step 3:Then go to theConnectiontab and clickLAN options.
Step 4:Under theProxy servercategory, uncheck theUse a proxy server for your LANbox.
Then verify theAutomatically detect settingsbox is checked.
Step 5:Then clickOKto save the change.
Then, check to see if the error still persists.
Thus, you might attempt to disable it.
Here are the instructions.
Step 1:Open themenuicon and selectOptions.
Go to theNetwork Proxysection.
Step 2:SelectSettings.
Go to theConfigure Proxy access to the Internetsection and select theNo proxyoption.
Step 3:PressOKand kickstart your net net app.
Method 5: Update Firefox
The last method for you is to update the Firefox web client.
The steps are as follows:
Step 1:Launch your Firefox online window.
Go to the top right corner to click themenuicon.
Step 2:ClickHelpand selectAbout Firefox.
You will be taken to theFirefox versionscreen.
Your web app will be updated automatically if need be.
Step 3:Restart the web app and see if the has been fixed after updating Firefox.