1) Windscribe VPN - https://windscribe.com/ Need registration on any email. 10Gb monthly limit on a free account.
Add-on v.0.1.61 for FF52 - https://www.upload.ee/files/11196640/windscribe-0.1.61-fx.xpi.html (versions newer 0.1.61 do not work in FF52).
Note: in about:config parameter "network.cookie.cookieBehavior" should be set to 0, 1 or 3.
Also available client for WinXP - https://assets.windscribe.com/desktop/win/Windscribe_1.80.exe
2) Browsec VPN.
Add-on v.3.16.16.2 for FF52 - https://www.upload.ee/files/11196624/browsec-3.16.16.2_unofficial.xpi.html
No registration required. Four countries are available. Unoficial build - no banner, no google-analytics.
Note: in about:config parameter "network.cookie.cookieBehavior" should be set to 0, 1 or 3.