Confluence editor load slower with large page with Firefox

STEP TO REPLICATE THE ISSUE

1. Create a page in Confluence with large content (about 20 to 50 page when exporting to WORD document)
2. Save the page and open the editor again in Firefox ESR 52.6.0 (64-bit).

EXPECTED RESULT
The editor will load the content immediately after user click Edit.

ACTUAL RESULT
The editor is loading close to 15 second before displaying the content after user click Edit. Notice the ...Connecting message at the bottom of the editor and it load close to 15 second while editing in Chrome load the content in the editor immediately.

NOTE

• The page editor load close to 15 second after user click Edit in Firefox ESR 52.6.0 (64-bit) browser.
• The page editor seems to load much faster (5 second) in Firefox Quantum 58.0.2 (64-bit) browser