I am using Firefox 66.0.3, addon version 3.4.3, Windows 7 64bit.
I forgot to mention that it happens not every time.
You can contact me at alexjzx (@) gmail.com.
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Add-on is really cool and it does the job well, but:
1) It takes too much time to load tabs, about 2-3 mins for ~60 tabs
2) Until it is loaded, you re not gonna to do anything to your tabs, because it will load tabs state in 3 mins and will erase all your progress(e.g. newly opened tabs)
3) Thus, it slows down starting your work with the browser.
For the rest, this addon is great and highly usable.