The flickering zoom is the result of a bug introduced in Version 14.0.

This problem can be avoided by setting "browser.zoom.siteSpecific" to "false" in Firefox's about:config preferences.

Version 14.1 fixes this bug and has been submitted for review and release (probably will take a few days).

With Version 14.1, setting "browser.zoom.siteSpecific" to "false" is still recommended, but is only necessary for the following features:

- per-tab zoom mode
- per-tab zoom when viewing images
- per-subsite zoom