Make it possible to copy single filters within the same mail account. If
possible, append the word (copy) to the filter, e.g.: "Fred Flinststone"
becomes "Fred Flinststone (copy)". This behavior is not necessary when copying
filters across accounts. Optionally open the Edit Filter dialog for the newly
inserted filter (with the copy appended to the title).

Accepted. Would it be acceptable if the Edit dialog always opens after the copy
has been inserted (it could even do this _before_ it is inserted and could
delete the filter if the user chooses to click on [Cancel].

It will get its own button for clarity (Clone), so that it is more obvious than
the 2step operation of copy / paste. (Copying + pasting a single filter in the
same account could still trigger the same action)

(In reply to comment #0)
> append the word (copy) to the filter, e.g.: "Fred Flinststone"
> becomes "Fred Flinststone (copy)". This behavior is not necessary when copying
> filters across accounts. 

I think I would append "(copy)" even across accounts.  

> Optionally open the Edit Filter dialog for the newly
> inserted filter (with the copy appended to the title).

That would be nice

(In reply to comment #2)
> (In reply to comment #0)
> > append the word (copy) to the filter, e.g.: "Fred Flinststone"
> > becomes "Fred Flinststone (copy)". This behavior is not necessary when copying
> > filters across accounts. 
> 
> I think I would append "(copy)" even across accounts.  
> 
not so sure, a lot of people would use copy / cut / paste to migrate filters
and changing the names might be not desirable

> > Optionally open the Edit Filter dialog for the newly
> > inserted filter (with the copy appended to the title).
> 
> That would be nice
> 
I am on it. :)

Created an attachment (id=7736) [details]
2.1 prerelease 12

First prototype/ Note - there are only German and English locales included for
the button / interface so your filter list will crash in other languages. you
have been warned.

editing in the cloned filter, changing subject term/contents changes it in both
the original and the new/cloned filter

changing actions, for me, changed only the new/cloned filter.  so it correctly
did NOT change the original

Created an attachment (id=7737) [details]
2.1 prerelease 15

tried to fix the cloning of search terms - these are now deeper copies of their
counterparts.

Created an attachment (id=7738) [details]
2.1 prerelease 17

Added a textbox so one can modify string for appending to the name for the
cloned filter. If empty, the default localized string "en:(copy)" is used.
Empty strings are not allowed by design.

yet incomplete locales (will lead to problems with options dialog):
fr, it, nl, vi, zh-CN, ru

testable: en, de, ja, sv-SE

(In reply to comment #7)
> Created an attachment (id=7738) [details] [details]
> 2.1 prerelease 17
> 
> Added a textbox so one can modify string for appending to the name for the
> cloned filter. If empty, the default localized string "en:(copy)" is used.
> Empty strings are not allowed by design.

thanks. first go, worked like a champ. 
new filter is unenabled.
don't know if that is by design, but I have no opinion either way for enabled
vs unenabled

(In reply to comment #8)
> 
> thanks. first go, worked like a champ. 
> new filter is unenabled.
> don't know if that is by design, but I have no opinion either way for enabled
> vs unenabled
> 
thanks for testing - not enabling new filters is not by [my] design - I also
find this varies, and currently have no idea what logic Thunderbird applies
when creating new filters. It might be a little counter-intuitive for some
users.

Note that on all new / merge / clone Filter operations, when the edit filter
dialog is cancelled then the new filter is scrapped [by design].

setting to fixed.