From MozillaZine Knowledge Base

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If you try typing in a text box on certain sites before the page is finished loading the focus can be confused and not respond as if you're focused in a text field. Waiting for the page to finish loading or refreshing the page can solve the issue.

Multiple Windows

Opening multiple windows using a desktop or quick launch shortcut can cause a focus bug which breaks clipboard tasks, and other focus dependant tasks, especially in text fields (Bug 220900). Minimizing the windows and maximizing them will usually reset focus once this issue is encountered. To avoid seeing this issue altogether, launch new windows using the New Window menu item, or short cut (Ctrl+N) or add a New Window button to your toolbar.

It is also possible to install an extension that can be used to redefine the apostrophe and slash ('/') keys used to invoke the Find bar and change them to rarely typed characters.

Related bugs

• Bug 220900: Focus breaks, cut/copy/paste and other focus-dependent tasks broken
• Bug 320465 – [meta] Bugs where typing apostrophe (or other chars) in textarea activates Find Bar