Thank you all for kind suggestions! Finally I figured out, what was the issue. I’m using KDE and wanted to have dynamic zoom with certain key + mouse scroll, which was achieved via xbindkeys in KDE, because KDE directly does not support it (hopefully yet), besides - such dynamic zoom support with configurable keys via GUI exist already in XFCE via ALT-key (hold ALT and scroll mouse wheel, in some distros it is needed to turn on in first place under Assistive Technologies, there is also graphically possible to define the key, that is used with mouse scroll).
Then I discovered from ~/.xbindkeysrc that there were created example content and some of these examples were commented in and there I found binding of CTRL+F with another app, xterm in my case.
Solution:
- commented these lines out:
# set directly keycode (here control + f with my keyboard)
#"xterm"
# c:41 + m:0x4
# specify a mouse button
#"xterm"
# control + b:2
- then after saving that configuration file, restarted xbindkeys
killall xbindkeys && xbindkeys&
After that CTRL+F was working again as expected.
To respond the previous question - using the latest software available (updating frequently), including Firefox.