Are you talking about the built-in Firefox viewer or Adobe?

The built in one. My workaround is ok (open in Sumatra pdf viewer) but I just thought this sounds like a bug needing fixing

I called for more help. Others may know of this.

Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Safe mode fixed it (but is obv not a practical solution). Hmm

Oh and thanks for the help of course.

It could be the work of one of your add-ons. Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control>(Mac=<Command>)<Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

Yes of course. FindbarTweak is to blame; the developer is aware.

In fact I find that add-on so useful I'll keep opening in an external viewer (sumatra)

Thanks for the support, I feel something of a fool for not having tracked it down myself, hey ho.

Glad to help. Safe Surfing.