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Description of problem:
Wen you use copy and paste in a HTML mail under google mail (only tested there), and you have a picture in your signature, and you use Spellcheck, Firefox freezes when copy/pasting text (I will try to explain it a bit better in the How to reproduce" steps

Version-Release number of selected component (if applicable):
Firefox 52

Steps to Reproduce:
1. receive a mail with a signature that contains an image
2. Reply/Forward this mail
3. Enable Spell check (here English (United States)
4. start typing random characters to trigger red underlining of errors
5. copy and paste a word from what you typed in 4. at the end of the line

Actual results:
- The system hangs for several seconds (gets worse the more text is underlined and the more signatures are in the thread
- The pictures in all signatures disappear for the freeze time (as if they are reloaded
- The underlining of wrong words disappears for a few seconds and reappears

Expected results:
Copy and paste should not freeze the browser window (or reload the pictures in signatures, or reload the underlining of wrong words


Let me know if you need more info, or if you want to have the behavior recorded  in a video

I cannot reproduce it on version 68. Please reopen if that's still the case.