Please consider turning on send info to Mozilla before posting a question. This helps us help you.

How about this... Please uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved.

Then Install Firefox Version 57 Full Installer Final Release https://fileforum.betanews.com/detail/Mozilla-Firefox-for-Windows/1032985422/1 Site gets files 24hrs before up on Mozilla Servers. Used site 20yrs........ And installed from it today myself. Note : Legacy Extensions will be disabled and or removed.

Please let us know if this solved your issue or if need further assistance.

I wonder if it's a Windows 10 theme issue because on Windows 7, the line spacing seems pretty normal.

Do you use an unusual Text Size / DPI setting in your Windows display settings? The common ones are 100% (96dpi), 125% (120dpi), and 150% (144dpi).

(Firefox typically follows the Windows percentage, but can be set to a custom level. You probably didn't do that manually, but if you ever used the Theme Font & Size Changer extension, that might have done it. Of course, that should not survive a Refersh.)

Hello, Thanks for the very quick response. To Pkshadow's suggestion, already did that, no change.

To jscher2000, No I don't use unusual dpi / font settings at all. Windows 10, October 17th update is when this started. I have written many " comments" on their feedback. 
I doubt this is firefox at all. However, I thought I would post here to see if there are similar situations and possible solutions.
 thanks again

Made might fix : Repair System Registry and Files

(FOR DOING Above & Below - WINDOWS All OTHER VERSIONS - Use Search To Find Command Prompt ** Enter CMD.exe to Find COMMAND PROMPT Then Right Click & RUN AS ADMIN)

To run the SFC command use the Windows key + X keyboard shortcut to open the Power User menu and select Command Prompt (Admin). Type "SFC /SCANNOW" (without quotes or copy/paste) and press Enter. For more information on using SFC, please refer to the Microsoft KB: Use the System File Checker tool to repair missing or corrupted system files

Run the Windows Deployment Image Servicing and Management (DISM) and System File Checker (SFC) tools as Administrator. The DISM and SFC tools scan the integrity of the Windows image and all protected system files replaces corrupted, damaged and incorrect versions. (FOR DOING Above & Below - WINDOWS All OTHER VERSIONS - Use Search To Find Command Prompt ** Enter CMD.exe to Find COMMAND PROMPT Then Right Click & RUN AS ADMIN) Type "DISM /Online /Cleanup-Image /RestoreHealth" (without quotes or copy/paste) and press Enter.

Windows 10 To run the DISM command use the Windows key + X keyboard shortcut to open the Power User menu, then select Command Prompt (Admin). Type "DISM /Online /Cleanup-Image /RestoreHealth" (without quotes or copy/paste) and press Enter. While running DISM you may notice the process will appear to pause at 20%. After a few minutes the operation will continue.

Note : SFC fixes files locally using files found on your system. Note : DISM goes up to Microsoft for new ones.

coz321 said

Windows 10, October 17th update is when this started. I have written many " comments" on their feedback.

Do you want to share a screenshot of what it looks like? Or if you plan to install Firefox 57 this week, I guess we could wait and see how that looks.

jscher2000,

 Not sure if I can attach a screenshot here. I'll stumble around, maybe find a way.
 Since Firefox 57 is coming out, I think I'll try that first.

Pkshadow, Because of the October 17th update to windows and the miscalculations it caused on our software, Vectric Aspire, cncwrapper, and Mach3, I have clean installed windows 5 times, different versions, to check if it was the update to blame, which it was, concerning the severe miscalculations creating and running g-code. However, I will certainly take your advice, soon. At the moment, having only tested once, the miscalculations, "seem" to have been solved with the current updated install. I hope it was our nearly relentless feedback to Microsoft about the calculation errors that a fix was implimented! Guys, thanks again. I will certainly let you knoe if the issue has been solved.

Hey guts. Installed the new Firefox, same issue. This is definitely caused by windows.

However, I am getting a new system and installing windows 10 Th2 version 1511 for our business keeping that offline. This system will be for online work. I won't be satisfied until I get to the bottom of the issue but for now it is only somewhat irritating. 

I thank you guys for the helpful responses, although any of the common possible fixes do not work.

 As mentioned, Microsoft, not unexpectedly, pushes out updates  sometimes ignoring possible conflicts. I really love Linux :)
 thanks again. If I find the issue I will certainly report back. Oh, I have attached a screenshot of one of the right click menus, which also reveals I apparently make typos.

Could you try this custom style rule in a userChrome.css file.

#contentAreaContextMenu menuitem, 
#contentAreaContextMenu menu, 
#contentAreaContextMenu menuseparator {
  height: 19.2px !important;
  min-height: 0 !important;
  max-height: 19.2px !important;
  line-height: 16.8px !important;
}

Those are the dimensions I have so it doesn't change anything on mine, but maybe it will help with yours?

If you have not already created a userChrome.css for some other UI modification purpose, this page will guide you: https://www.userchrome.org/how-create-userchrome-css.html

It might also be an issue with the font being used.