Which I at first thought had to do with not getting the variable right--- but a little detective work convinced me that what it couldn't find were the two scripts.
IMO you are wrong, if you don't have script in place, the shell script will say

./check.sh: line 3: firstscript: command not found
./check.sh: line 4: secondscript: command not found
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IMO you have problem with arguments only, and might be some problems with the absolute path of the first and second script(as other monks mentioned).
generally if you are dealing with filenames in shell scripts, just check it existence before using it for further processing.
you can use something like

       -e file
              True if file exists.
       -f file
              True if file exists and is a regular file.
       -r file
              True if file exists and is readable.
       -s file
              True if file exists and has a size greater than zero.
       -O file
              True if file exists and is owned by the effective user i
+d.
       -G file
              True if file exists and is owned by the effective group 
+id.
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and more of this can be seen in your shell manual(the above operators are from bash manual), use those effectively to figure out whether the file exists and continue only in such case. For the script use

       -x file
              True if file exists and is executable.
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before calling it from your shell script.

if I write your script (shown in OP), in bash, it will be like

#!/bin/bash
echo -e "received arguments >$*<"
filename=$1;
script1='fristscript'; # use absolute path
script2='firstscript'; # use absolute path
if [ -f $filename ] ; then
    echo -e "executing scripts $script1, $script2";
    if [ -x $script1 ]; then
        echo -e "executing $script1..."
        $script1 $filename
    else
        echo -e "$script1 not found";
    fi
    if [ -x $script2 ]; then
        echo -e "executing $script2..."
        $script2 $filename
    else
        echo -e "$script2 not found";
    fi
else
    echo -e "$filename is not readable or doesn't exist";
fi
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Hope this helps.
(again the above bash code can be written even more effectively, but that is not the point of discussion)