Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Try the version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

should have mentioned, I'd tried in safe mode and I still have the issue.

no themes to speak of and only a handful of add-ons.

All the text would normally only get selected if you press Ctrl+A or double-click the mouse button, accidentally or maybe a problem with the mouse.

Are you using a normal mouse or a touch pad?

If you use a touch pad then make sure not to activate any click events.

I'm using a standard mouse and mousepad. The fact that it didn't happen elsewhere made me wonder if some configuration setting (about:config) would impact this.

If it only happen in the location/address bar the you could try to toggle some browser.urlbar prefs to false, but you indicate that the search bar is affected as well

• browser.urlbar.autocomplete.enabled
• browser.urlbar.autoFill
• browser.urlbar.autoFill.typed

Try the version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

You can find the full version of the current Firefox release (42.0) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/