Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}acking-protection-firefox-desktop

Tried without add-ons enabled. Still the same issue.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

I'm not sure what you think in wrong with the Google translate page as the screenshot shows that translating works properly. Blinking cursors may not show in the screenshot. I'm not seeing the "Enter text" placeholder text in Firefox if I click the clear X button, but instead get a blinking cursor like when I initially open the translate page. The Inspector shows that the placeholder attribute of this textarea is empty and I only see a blinking cursor and entering new translate in the left textarea text works properly and shows the translation in the right textarea. It is quite possible that other browsers show their own default placeholder text in case the website doesn't provide it unlike Firefox.

Firefox users already know where to type their text. ;-)