From MozillaZine Knowledge Base
Some localized e-mail applications use non-standard reply indicators, such as Aw:, VS: or Svar:, instead of the traditional Re:. When answering to a message starting with AW:, Thunderbird does not recognize this to be a reply indicator and adds a Re: to the subject (becoming Re: AW:). You can avoid endless chains of reply indicators by setting Thunderbird to accept also other forms, which are then replaced while replying.
This happens by adding the following lines to your user.js file:
// Recognition of localized reply indicators user_pref("mailnews.localizedRe", "AW,Aw,Antwort,VS,Vs,SV,Sv,Svar");
You may want to change the value of the variable according to your own needs. Separate the elements of the list by commas and do not include the :. Note that the handling of this variable is case-sensitive.
This change only affects messages that arrive after you've modified your user.js file. To make it apply to all messages, follow these steps:
Some trivia: Often mistaken for being an abbreviation of "reply", "re" is the ablative singular form of "res, rei" (a Latin noun meaning "thing, matter"), the indicator Re: stands for "In this matter", or "With regard to".
The Clean Subject add-on allows further customization of the "Re:" and "Fwd:" additions to the subject line.