Just downloaded Arduino Version 1.8.9 and am using Files --> Preferences and adding "http://arduino.esp8266.com/stable/package_esp8266com_index.json" to the "Additional Boards Manager URLs:"
When going to the Board Manager, "esp8266 by esp8266 community" does not show up in the list ????
What happened to it as I need it and how to fix?
Thanks,
I use "Arduino 1.8.4" with "esp8266 by ESP8266 Community version 2.3.0" at work with the following settings and it works 100%:
Board "Generic ESP8266 Module"
Flash Mode "QIO"
Flash Size "4M(1M SPIFFS)"
CPU Frequency "80MHz"
Debug Port "Disabled"
Debug Level "None"
Reset Method "ck"
Upload Speed" 115200"
This works perfect on the same device that I am having on my home computer (See working.png)
However at my home computer, I am using the latest versions "Arduino 1.8.9" with "esp8266 by ESP8266 Community version 2.5.1". What I see is that the LED on the ESP8266 blinks every few seconds instead of flashing during the flashing process. There seems to be many other settings in the Tools menu (see Not Working.png).
The output window at the bottom of the Arduino program also seems to be different during flashing of the ESP8266. I don't know if the new versions of Arduino or the new versions of the "esp8266 by ESP8266 Community version 2.5.1" is causing the issues?
Any ideas???

Well, I found the source of the problem.
There is a problem when using the "Generic ESP8266 Module" and the "esp8266 by ESP8266 Community Version 2.5.1", from http://arduino.esp8266.com/stable/package_esp8266com_index.json.
I removed 2.5.1 and installed Version 2.5.0 and it works perfect.
Therefore ESP8266 Community, please fix your latest version!!!
Seems like its not the board problem, its Arduino problem. So we just wait until its fixed?
Please see this post
Then ensure you are or were not affected.