Firefox 42+ has a Tracking Protection feature that is enabled by default in Private Browsing mode. When you are in private browsing mode then Firefox can show a shield icon at the left end of the location/address bar that some content is affected.

You can disable this feature in "Options/Preferences > Privacy" or via the about:privatebrowsing page that get when you open a New Private Window.

• Options/Preferences > Privacy: "Use Tracking Protection in Private Windows"

• https://support.mozilla.org/kb/tracking-protection-firefox
• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

Your suggestion did not work, but thank you for your response.

I don't use Private Browsing with FF.

Despite that and despite not seeing the shield icon, I unchecked "Options/Preferences > Privacy: "Use Tracking Protection in Private Windows" as you suggested.

I then closed FF, reopened and clicked "Restore Previous Session".

The "Follow" button on Twitter appeared for about 1 second and promptly disappeared again.

I never had this problem before upgrading to FF 43, and as mentioned previously I can see the Follow button on twitter with Chrome.

Do you (or anyone else) have a suggestion?

I'm amazed if I'm the only one posting about this.

Windows 7. Laptop.

Do you have any content blocking extensions like Adblock Plus or Ghostery?

I've assumed that you had already checked that and Tracking Protection is a new feature and you may not be aware of that it can hide content like buttons created by a script that is blocked.

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

"Do you have any content blocking extensions like Adblock Plus or Ghostery?

I've assumed that you had already checked that..."

Yes - prior to my 1st post I turned off Adblock Plus for the twitter page I was on, and then with it off for Twitter.

No change.

However, I just disabled it totally and now see the "Follow" button on Twitter after restarting FF.

So it looks like Adblock Plus has to be disabled every time I want to Follow/Unfollow on Twitter.

That is a monster hassle, but I love Adblock Plus so I guess I'll have to do this until FF can fix whatever in Version 43 (.0.4 currently) is causing this.

Thank you for your suggestions/help. Appreciate it.

You can try this extension instead or ask advice at the ABP forum site.

• uBlock Origin: https://addons.mozilla.org/firefox/addon/ublock-origin/

• https://adblockplus.org/forum/

Thanks, will check those.

I also find that now the start/pause Youtube button is no longer visible although in this case it operates as normal if I click in the space where I know that button is located. It's just invisible now.

I assume a FF 3/Adblock Plus conflict as well, as never had this previously..

Solved.

I updated FF but hadn't considered that there might be an update for Adblock Plus.

Just updated it and now FF is working as previously with Twitter and Youtube.

And will try out your suggestion of uBlock to see if I like that better.