@WS64 Try running Firefox in safe mode.
IIRC I saw a similar problem with a file manager product, the problem was not firefox itself but an extension or plug-in. It was some sort of 'close embrace', firefox (via the ext/pi) it would 'lock' a resource that the file manager had allocated, hence it never got de-allocated.
Dunno whether that makes any sense in the context of calibre - but maybe worth a try
BR
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