You may have switched on caret browsing.

• http://kb.mozillazine.org/accessibility.browsewithcaret

You can press press F7 (on Mac: fn + F7) to toggle caret browsing on/off.

• Tools > Options > Advanced : General: Accessibility: [ ] "Always use the cursor keys to navigate within pages"

• http://kb.mozillazine.org/Scrolling_with_arrow_keys_no_longer_works
• http://kb.mozillazine.org/Accessibility_features_of_Firefox

thx for your fast reply. It doesn't solve my problem. I can't select any entry (search area, url area; see attached files) with my arrow buttons.

I believe the preference you want is this.
accessibility.browsewithcaret = false

accessibility.warn_on_browsewithcaret only controls the warning message when you are enabling Caret browsing.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

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

same behavior in safe mode and ....browsewithcaret = false

(

additional information:

at the start of firefox the selection by arrow keys works for a few seconds, then it stops suddenly

moreover the accessibility.browsewithcaret sets back to true itselfes

Is that only a problem with those two drop down list or also with selection lists on web pages?

also selction lists on any web pages

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.org/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

new profile and alle works fine, thx a lot!

You're welcome