Could you test with Nightly, it seems to be fixed. http://nightly.mozilla.org/
I still see the issue in nightly. The "Big text" jumps down about 30 pixels after smoothly growing, then jumps back up right before it starts shrinking again. For anyone looking at the "Reporter's testcase" attachment (thanks for making it, Loic), it won't run from bugzilla in nightly due to a mixed content error. Downloading then running from the filesystem works fine.
Incidentally, I'm also seeing "Big text" intermittently jumping during the transition. It appears to be jumping by the same proportion as the consistent final jump when this happens, then it returns to its normal path immediately. I'm not sure if that's a separate issue or not, and either way I don't know how to reproduce it 100% of the time. But if you let my test case run for 30 seconds you're likely to witness it at least once.
I can reproduce the jumping if change zoom level of text only. Steps to Reproduce: 1. Start Firefox with Clean profile 2. Open attachment 751452 [details] 3. Click "mixed-content-blocked-notification-icon" and Choose "Disable Protection on This Page" in dropdowm menu 4. Make sure "Zoom Text Only" mode, Alt > View > Zoom > Check "Zoom Text Only" 5. Zoom out(Ctrl -) or Zoom in(Ctrl +)
And Reproduced in Firefox4 - Nightly24.0a1
Status: UNCONFIRMED → NEW
Ever confirmed: true
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