How can I use variable in AppleScript for name of browser?
That won’t work because the target for an application tell statement is needed at compile time to load its scripting dictionary.  An application’s terminology is defined however the developer wants it, so anything similar between applications is more of an accident than any kind of convention.

The closest you are going to get would be to programmatically build a script as a text string and use it with a run script statement, since the script in the string is compiled when it is used:

set theApp to "Safari"
set theScript to "tell application \"" &amp; theApp &amp; "\" to get tabs of window 1"
run script theScript