v2.0.0

• now requires at least Thunderbird 52
• added toolbar button and menuitem for sign rules (#78)
• added option to try to read non RFC compliant Authentication-Results header (#79, #71, #49)
• fixed incompatibility with Thunderbird 57/59/60 (#93, #94, #95, #96, #97, #99)
• fixed favicons not being shown if the CardBook add-on is installed
• fixed "Add must be signed exception" button being disabled if wrong signer is only a warning
• updated default rules and favicons