JIRA Agile Kanban board shows a "Unresponsive script" warning in Firefox

Precondition

1. JIRA and JIRA Agile Installed.
2. Agile Kanban board exists with lots of issues (1500+)

Steps to reproduce

1. Open Kanban board with lots of issues in Firefox, e.g. https://jira.atlassian.com/browse/SRCTREEWIN

Result

• Page is greyed out
• Warning: Unresponsive script is shown

Expected Result

• Waiting spinner is shown
• Page loads without any warnings

My system configuration
I run Firefox on:

• Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz
• 16 GB ram

Environment
JIRA version: v6.5-m05#65001
JIRA Agile v6.7.4

Additional info
Please refer to the attached screenshots and profiling information from Chrome (file SourceTreeRenderingCPU-20150804T115510.cpuprofile).