Added screen shot. (Wordpress.com admin editing image caption.) Same thing happens with two different forums based on myBB.de

With such text areas there is usually some JavaScript active that tries to keep the size of this field as such that all the content fits (i.e. it auto-resizes). In your case it sounds that this isn't working properly. This could be a font issue where a larger font is used then expected or you are blocking JavaScript that is responsible.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case Content Blocking is blocking content.

• click the shield icon for more details and possibly disable the protection

You can check the Web Console about what content is blocked

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/tracking-protection
• https://support.mozilla.org/en-US/kb/content-blocking

You can also check the Web Console for related messages.

• "3-bar" menu button or Tools -> Web Developer

Nope. None of the above. There was no shield icon. I have Adblock plus and turned off blocking for the site. No change after reload. All (few) extensions are up to date. ABP is the only one supposed to do manipulation of pages. No output in the web console.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

You are currently using Firefox from the Ubuntu repositories. You can test if this also happens with the official release or try a new profile.

• https://www.mozilla.org/en-US/firefox/all/
• https://support.mozilla.org/en-US/kb/Profiles

Thanks! Works in Safe Mode.

Found the culprit: "Grammar and Spell Checker - LanguageTool"

Nice tool, but I can do without. Disabling it on the affected sites is enough.

You're welcome and thanks for reporting what extension caused it.